title: git submodule not updating to parent branch
body: <p>I have the following in my <code>.gitmodules</code>:</p>
  <pre><code>[submodule &quot;django&quot;]
      path = django
      url = git@gitlab.******.com:path/to/django.git
      branch = .
  </code></pre>
  <p>This file is identical in my <code>master</code> and <code>uat</code> branches.</p>
  <p>Regarding the branch, from the <a href="https://git-scm.com/docs/gitmodules" rel="nofollow noreferrer">docs</a></p>
  <blockquote>
  <p>A special value of . is used to indicate that the name of the branch
  in the submodule should be the same name as the current branch in the
  current repository.</p>
  </blockquote>
  <p>Here's what I'm doing:</p>
  <pre><code>git checkout master &amp;&amp; git submodule sync &amp;&amp; git submodule update &amp;&amp; git submodule status
  </code></pre>
  <p>Which gives me:</p>
  <pre><code>f6d808827aca47984d495e0fe2403dd41f042e40 django (heads/master)
  </code></pre>
  <p>I switch the parent to <code>uat</code> branch:</p>
  <pre><code>git checkout uat &amp;&amp; git submodule sync &amp;&amp; git submodule update &amp;&amp; git submodule status
  </code></pre>
  <p>This outputs the exact same ref:</p>
  <pre><code>f6d808827aca47984d495e0fe2403dd41f042e40 django (heads/master)
  </code></pre>
  <p>I expect it to be pointing at a commit on the <code>uat</code> branch.</p>
  <p>Why isn't it?</p>