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<div id="header">
<h1>How to separate topic branches</h1>
<span id="revdate"></span>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p>This text was originally a footnote to a discussion about the
behaviour of the git diff commands.</p></div>
<div class="paragraph"><p>Often I find myself doing that [running diff against something other
than HEAD] while rewriting messy development history.  For example, I
start doing some work without knowing exactly where it leads, and end
up with a history like this:</p></div>
<div class="literalblock">
<div class="content">
<pre><code>    "master"
o---o
     \                    "topic"
      o---o---o---o---o---o</code></pre>
</div></div>
<div class="paragraph"><p>At this point, "topic" contains something I know I want, but it
contains two concepts that turned out to be completely independent.
And often, one topic component is larger than the other.  It may
contain more than two topics.</p></div>
<div class="paragraph"><p>In order to rewrite this mess to be more manageable, I would first do
"diff master..topic", to extract the changes into a single patch, start
picking pieces from it to get logically self-contained units, and
start building on top of "master":</p></div>
<div class="literalblock">
<div class="content">
<pre><code>$ git diff master..topic &gt;P.diff
$ git checkout -b topicA master
... pick and apply pieces from P.diff to build
... commits on topicA branch.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>      o---o---o
     /        "topicA"
o---o"master"
     \                    "topic"
      o---o---o---o---o---o</code></pre>
</div></div>
<div class="paragraph"><p>Before doing each commit on "topicA" HEAD, I run "diff HEAD"
before update-index the affected paths, or "diff --cached HEAD"
after.  Also I would run "diff --cached master" to make sure
that the changes are only the ones related to "topicA".  Usually
I do this for smaller topics first.</p></div>
<div class="paragraph"><p>After that, I&#8217;d do the remainder of the original "topic", but
for that, I do not start from the patchfile I extracted by
comparing "master" and "topic" I used initially.  Still on
"topicA", I extract "diff topic", and use it to rebuild the
other topic:</p></div>
<div class="literalblock">
<div class="content">
<pre><code>$ git diff -R topic &gt;P.diff ;# --cached also would work fine
$ git checkout -b topicB master
... pick and apply pieces from P.diff to build
... commits on topicB branch.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>                        "topicB"
       o---o---o---o---o
      /
     /o---o---o
    |/        "topicA"
o---o"master"
     \                    "topic"
      o---o---o---o---o---o</code></pre>
</div></div>
<div class="paragraph"><p>After I am done, I&#8217;d try a pretend-merge between "topicA" and
"topicB" in order to make sure I have not missed anything:</p></div>
<div class="literalblock">
<div class="content">
<pre><code>$ git pull . topicA ;# merge it into current "topicB"
$ git diff topic
                        "topicB"
       o---o---o---o---o---* (pretend merge)
      /                   /
     /o---o---o----------'
    |/        "topicA"
o---o"master"
     \                    "topic"
      o---o---o---o---o---o</code></pre>
</div></div>
<div class="paragraph"><p>The last diff better not to show anything other than cleanups
for cruft.  Then I can finally clean things up:</p></div>
<div class="literalblock">
<div class="content">
<pre><code>$ git branch -D topic
$ git reset --hard HEAD^ ;# nuke pretend merge</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>                        "topicB"
       o---o---o---o---o
      /
     /o---o---o
    |/        "topicA"
o---o"master"</code></pre>
</div></div>
</div>
</div>
</div>
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Last updated
 2025-06-13 22:14:31 UTC
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