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1 <!-- vim: set filetype=docbkxml shiftwidth=2 autoindent expandtab tw=77 : -->
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2
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3 <chapter id="chap:tour-merge">
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4 <title>A tour of Mercurial: merging work</title>
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5
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6 <para>We've now covered cloning a repository, making changes in a
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7 repository, and pulling or pushing changes from one repository
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8 into another. Our next step is <emphasis>merging</emphasis>
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9 changes from separate repositories.</para>
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10
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11 <sect1>
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12 <title>Merging streams of work</title>
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13
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14 <para>Merging is a fundamental part of working with a distributed
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15 revision control tool.</para>
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16 <itemizedlist>
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17 <listitem><para>Alice and Bob each have a personal copy of a
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18 repository for a project they're collaborating on. Alice
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19 fixes a bug in her repository; Bob adds a new feature in
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20 his. They want the shared repository to contain both the
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21 bug fix and the new feature.</para>
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22 </listitem>
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23 <listitem><para>I frequently work on several different tasks for
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24 a single project at once, each safely isolated in its own
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25 repository. Working this way means that I often need to
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26 merge one piece of my own work with another.</para>
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27 </listitem></itemizedlist>
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28
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29 <para>Because merging is such a common thing to need to do,
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30 Mercurial makes it easy. Let's walk through the process. We'll
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31 begin by cloning yet another repository (see how often they
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32 spring up?) and making a change in it.</para>
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33
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34 &interaction.tour.merge.clone;
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35
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36 <para>We should now have two copies of
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37 <filename>hello.c</filename> with different contents. The
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38 histories of the two repositories have also diverged, as
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39 illustrated in figure <xref
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40 linkend="fig:tour-merge:sep-repos"/>.</para>
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41
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42 &interaction.tour.merge.cat;
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43
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44 <informalfigure id="fig:tour-merge:sep-repos">
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45 <mediaobject>
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46 <imageobject><imagedata fileref="tour-merge-sep-repos"/></imageobject>
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47 <textobject><phrase>XXX add text</phrase></textobject>
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48 <caption><para>Divergent recent histories of the <filename
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49 class="directory">my-hello</filename> and <filename
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50 class="directory">my-new-hello</filename>
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51 repositories</para></caption>
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52 </mediaobject>
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53 </informalfigure>
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54
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55 <para>We already know that pulling changes from our <filename
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56 class="directory">my-hello</filename> repository will have no
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57 effect on the working directory.</para>
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58
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59 &interaction.tour.merge.pull;
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60
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61 <para>However, the <command role="hg-cmd">hg pull</command>
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62 command says something about <quote>heads</quote>.</para>
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63
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64 <sect2>
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65 <title>Head changesets</title>
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66
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67 <para>A head is a change that has no descendants, or children,
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68 as they're also known. The tip revision is thus a head,
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69 because the newest revision in a repository doesn't have any
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70 children, but a repository can contain more than one
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71 head.</para>
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72
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73 <informalfigure id="fig:tour-merge:pull">
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74 <mediaobject><imageobject><imagedata
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75 fileref="tour-merge-pull"/></imageobject><textobject><phrase>XXX
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76 add text</phrase></textobject>
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77 <caption><para>Repository contents after pulling from
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78 <filename class="directory">my-hello</filename> into
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79 <filename
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80 class="directory">my-new-hello</filename></para></caption>
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81 </mediaobject>
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82 </informalfigure>
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83
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84 <para>In figure <xref linkend="fig:tour-merge:pull"/>, you can
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85 see the effect of the pull from <filename
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86 class="directory">my-hello</filename> into <filename
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87 class="directory">my-new-hello</filename>. The history that
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88 was already present in <filename
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89 class="directory">my-new-hello</filename> is untouched, but
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90 a new revision has been added. By referring to figure <xref
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91 linkend="fig:tour-merge:sep-repos"/>, we can see that the
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92 <emphasis>changeset ID</emphasis> remains the same in the new
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93 repository, but the <emphasis>revision number</emphasis> has
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94 changed. (This, incidentally, is a fine example of why it's
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95 not safe to use revision numbers when discussing changesets.)
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96 We can view the heads in a repository using the <command
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97 role="hg-cmd">hg heads</command> command.</para>
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98
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99 &interaction.tour.merge.heads;
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100
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101 </sect2>
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102 <sect2>
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103 <title>Performing the merge</title>
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104
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105 <para>What happens if we try to use the normal <command
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106 role="hg-cmd">hg update</command> command to update to the
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107 new tip?</para>
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108
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109 &interaction.tour.merge.update;
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110
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111 <para>Mercurial is telling us that the <command role="hg-cmd">hg
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112 update</command> command won't do a merge; it won't update
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113 the working directory when it thinks we might be wanting to do
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114 a merge, unless we force it to do so. Instead, we use the
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115 <command role="hg-cmd">hg merge</command> command to merge the
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116 two heads.</para>
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117
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118 &interaction.tour.merge.merge;
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119
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120 <informalfigure id="fig:tour-merge:merge">
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121
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122 <mediaobject><imageobject><imagedata
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123 fileref="tour-merge-merge"/></imageobject><textobject><phrase>XXX
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124 add text</phrase></textobject>
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125 <caption><para>Working directory and repository during
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126 merge, and following commit</para></caption>
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127 </mediaobject>
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128 </informalfigure>
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129
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130 <para>This updates the working directory so that it contains
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131 changes from <emphasis>both</emphasis> heads, which is
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132 reflected in both the output of <command role="hg-cmd">hg
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133 parents</command> and the contents of
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134 <filename>hello.c</filename>.</para>
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135
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136 &interaction.tour.merge.parents;
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137
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138 </sect2>
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139 <sect2>
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140 <title>Committing the results of the merge</title>
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141
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142 <para>Whenever we've done a merge, <command role="hg-cmd">hg
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143 parents</command> will display two parents until we <command
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144 role="hg-cmd">hg commit</command> the results of the
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145 merge.</para>
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146
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147 &interaction.tour.merge.commit;
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148
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149 <para>We now have a new tip revision; notice that it has
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150 <emphasis>both</emphasis> of our former heads as its parents.
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151 These are the same revisions that were previously displayed by
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152 <command role="hg-cmd">hg parents</command>.</para>
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153
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154 &interaction.tour.merge.tip;
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155
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156 <para>In figure <xref
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157 linkend="fig:tour-merge:merge"/>, you can see a
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158 representation of what happens to the working directory during
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159 the merge, and how this affects the repository when the commit
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160 happens. During the merge, the working directory has two
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161 parent changesets, and these become the parents of the new
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162 changeset.</para>
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163
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164 </sect2>
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165 </sect1>
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166 <sect1>
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167 <title>Merging conflicting changes</title>
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168
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169 <para>Most merges are simple affairs, but sometimes you'll find
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170 yourself merging changes where each modifies the same portions
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171 of the same files. Unless both modifications are identical,
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172 this results in a <emphasis>conflict</emphasis>, where you have
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173 to decide how to reconcile the different changes into something
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174 coherent.</para>
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175
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176 <informalfigure>
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177
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178 <mediaobject id="fig:tour-merge:conflict">
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179 <imageobject><imagedata fileref="tour-merge-conflict"/></imageobject>
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180 <textobject><phrase>XXX add text</phrase></textobject>
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181 <caption><para>Conflicting changes to a
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182 document</para></caption> </mediaobject>
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183 </informalfigure>
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184
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185 <para>Figure <xref linkend="fig:tour-merge:conflict"/> illustrates
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186 an instance of two conflicting changes to a document. We
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187 started with a single version of the file; then we made some
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188 changes; while someone else made different changes to the same
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189 text. Our task in resolving the conflicting changes is to
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190 decide what the file should look like.</para>
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191
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192 <para>Mercurial doesn't have a built-in facility for handling
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193 conflicts. Instead, it runs an external program called
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194 <command>hgmerge</command>. This is a shell script that is
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195 bundled with Mercurial; you can change it to behave however you
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196 please. What it does by default is try to find one of several
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197 different merging tools that are likely to be installed on your
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198 system. It first tries a few fully automatic merging tools; if
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199 these don't succeed (because the resolution process requires
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200 human guidance) or aren't present, the script tries a few
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201 different graphical merging tools.</para>
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202
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203 <para>It's also possible to get Mercurial to run another program
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204 or script instead of <command>hgmerge</command>, by setting the
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205 <envar>HGMERGE</envar> environment variable to the name of your
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206 preferred program.</para>
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207
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208 <sect2>
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209 <title>Using a graphical merge tool</title>
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210
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211 <para>My preferred graphical merge tool is
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212 <command>kdiff3</command>, which I'll use to describe the
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213 features that are common to graphical file merging tools. You
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214 can see a screenshot of <command>kdiff3</command> in action in
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215 figure <xref linkend="fig:tour-merge:kdiff3"/>. The kind of
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216 merge it is performing is called a <emphasis>three-way
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217 merge</emphasis>, because there are three different versions
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218 of the file of interest to us. The tool thus splits the upper
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219 portion of the window into three panes:</para>
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220 <itemizedlist>
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221 <listitem><para>At the left is the <emphasis>base</emphasis>
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222 version of the file, i.e. the most recent version from
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223 which the two versions we're trying to merge are
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224 descended.</para>
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225 </listitem>
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226 <listitem><para>In the middle is <quote>our</quote> version of
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227 the file, with the contents that we modified.</para>
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228 </listitem>
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229 <listitem><para>On the right is <quote>their</quote> version
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230 of the file, the one that from the changeset that we're
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231 trying to merge with.</para>
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232 </listitem></itemizedlist>
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233 <para>In the pane below these is the current
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234 <emphasis>result</emphasis> of the merge. Our task is to
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235 replace all of the red text, which indicates unresolved
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236 conflicts, with some sensible merger of the
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237 <quote>ours</quote> and <quote>theirs</quote> versions of the
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238 file.</para>
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239
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240 <para>All four of these panes are <emphasis>locked
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241 together</emphasis>; if we scroll vertically or horizontally
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242 in any of them, the others are updated to display the
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243 corresponding sections of their respective files.</para>
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244
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245 <informalfigure id="fig:tour-merge:kdiff3">
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246 <mediaobject><imageobject><imagedata
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247 fileref="kdiff3"/></imageobject><textobject><phrase>XXX
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248 add text</phrase></textobject>
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249 <caption><para>Using <command>kdiff3</command> to merge
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250 versions of a file</para></caption>
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251 </mediaobject>
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252 </informalfigure>
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253
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254 <para>For each conflicting portion of the file, we can choose to
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255 resolve the conflict using some combination of text from the
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256 base version, ours, or theirs. We can also manually edit the
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257 merged file at any time, in case we need to make further
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258 modifications.</para>
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259
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260 <para>There are <emphasis>many</emphasis> file merging tools
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261 available, too many to cover here. They vary in which
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262 platforms they are available for, and in their particular
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263 strengths and weaknesses. Most are tuned for merging files
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264 containing plain text, while a few are aimed at specialised
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265 file formats (generally XML).</para>
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266
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267 </sect2>
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268 <sect2>
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269 <title>A worked example</title>
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270
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271 <para>In this example, we will reproduce the file modification
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272 history of figure <xref linkend="fig:tour-merge:conflict"/>
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273 above. Let's begin by creating a repository with a base
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274 version of our document.</para>
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275
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276 &interaction.tour-merge-conflict.wife;
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277
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278 <para>We'll clone the repository and make a change to the
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279 file.</para>
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280
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281 &interaction.tour-merge-conflict.cousin;
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282
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283 <para>And another clone, to simulate someone else making a
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284 change to the file. (This hints at the idea that it's not all
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285 that unusual to merge with yourself when you isolate tasks in
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286 separate repositories, and indeed to find and resolve
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287 conflicts while doing so.)</para>
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288
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289 &interaction.tour-merge-conflict.son;
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290
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291 <para>Having created two
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292 different versions of the file, we'll set up an environment
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293 suitable for running our merge.</para>
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294
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295 &interaction.tour-merge-conflict.pull;
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296
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297 <para>In this example, I won't use Mercurial's normal
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298 <command>hgmerge</command> program to do the merge, because it
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299 would drop my nice automated example-running tool into a
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300 graphical user interface. Instead, I'll set
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301 <envar>HGMERGE</envar> to tell Mercurial to use the
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302 non-interactive <command>merge</command> command. This is
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303 bundled with many Unix-like systems. If you're following this
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304 example on your computer, don't bother setting
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305 <envar>HGMERGE</envar>.</para>
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306
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307 <para><emphasis role="bold">XXX FIX THIS
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308 EXAMPLE.</emphasis></para>
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309
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310 &interaction.tour-merge-conflict.merge;
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311
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312 <para>Because <command>merge</command> can't resolve the
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313 conflicting changes, it leaves <emphasis>merge
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314 markers</emphasis> inside the file that has conflicts,
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315 indicating which lines have conflicts, and whether they came
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316 from our version of the file or theirs.</para>
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317
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318 <para>Mercurial can tell from the way <command>merge</command>
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319 exits that it wasn't able to merge successfully, so it tells
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320 us what commands we'll need to run if we want to redo the
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321 merging operation. This could be useful if, for example, we
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322 were running a graphical merge tool and quit because we were
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323 confused or realised we had made a mistake.</para>
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324
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325 <para>If automatic or manual merges fail, there's nothing to
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326 prevent us from <quote>fixing up</quote> the affected files
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327 ourselves, and committing the results of our merge:</para>
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328
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329 &interaction.tour-merge-conflict.commit;
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330
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331 </sect2>
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332 </sect1>
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333 <sect1 id="sec:tour-merge:fetch">
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334 <title>Simplifying the pull-merge-commit sequence</title>
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335
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336 <para>The process of merging changes as outlined above is
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337 straightforward, but requires running three commands in
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338 sequence.</para>
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339 <programlisting>
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340 hg pull hg merge hg commit -m 'Merged remote changes'
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341 </programlisting>
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342 <para>In the case of the final commit, you also need to enter a
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343 commit message, which is almost always going to be a piece of
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344 uninteresting <quote>boilerplate</quote> text.</para>
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345
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346 <para>It would be nice to reduce the number of steps needed, if
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347 this were possible. Indeed, Mercurial is distributed with an
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348 extension called <literal role="hg-ext">fetch</literal> that
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349 does just this.</para>
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350
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351 <para>Mercurial provides a flexible extension mechanism that lets
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352 people extend its functionality, while keeping the core of
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353 Mercurial small and easy to deal with. Some extensions add new
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354 commands that you can use from the command line, while others
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355 work <quote>behind the scenes,</quote> for example adding
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356 capabilities to the server.</para>
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357
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358 <para>The <literal role="hg-ext">fetch</literal> extension adds a
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359 new command called, not surprisingly, <command role="hg-cmd">hg
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360 fetch</command>. This extension acts as a combination of
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361 <command role="hg-cmd">hg pull</command>, <command
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362 role="hg-cmd">hg update</command> and <command
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363 role="hg-cmd">hg merge</command>. It begins by pulling
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364 changes from another repository into the current repository. If
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365 it finds that the changes added a new head to the repository, it
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366 begins a merge, then commits the result of the merge with an
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367 automatically-generated commit message. If no new heads were
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368 added, it updates the working directory to the new tip
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369 changeset.</para>
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370
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371 <para>Enabling the <literal role="hg-ext">fetch</literal>
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372 extension is easy. Edit your <filename
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373 role="special">.hgrc</filename>, and either go to the <literal
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374 role="rc-extensions">extensions</literal> section or create an
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375 <literal role="rc-extensions">extensions</literal> section. Then
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376 add a line that simply reads <quote><literal>fetch
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377 </literal></quote>.</para>
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378 <programlisting>
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379 [extensions] fetch =
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380 </programlisting>
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381 <para>(Normally, on the right-hand side of the
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382 <quote><literal>=</literal></quote> would appear the location of
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383 the extension, but since the <literal
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384 role="hg-ext">fetch</literal> extension is in the standard
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385 distribution, Mercurial knows where to search for it.)</para>
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386
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387 </sect1>
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388 </chapter>
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389
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390 <!--
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391 local variables:
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392 sgml-parent-document: ("00book.xml" "book" "chapter")
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393 end:
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394 -->
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