title: Why would 'git submodule update' skip a submodule?
body: <p>I have a git repo with a single submodule <code>sub/x</code>.  (This submodule does not contain any submodules of its own.)</p>
  <p>In the superproject's repo, the output of <code>git status</code> shows the following (unstaged) modification</p>
  <pre><code>modified:   sub/x (new commits)
  </code></pre>
  <p>If I now run</p>
  <pre><code>git submodule update
  </code></pre>
  <p>...on the superproject, the following line gets printed to the terminal (and nothing more):</p>
  <pre><code>Skipping submodule 'sub/x'
  </code></pre>
  <p>After this, the output of <code>git status</code> on the superproject remains as shown above, unchanged.</p>
  <p>(Ditto if I add <code>--init</code> to the <code>git submodule update</code> command.)</p>
  <p><strong>Q:</strong> How can I determine why <code>git submodule update [--init]</code> skips the <code>sub/x</code> submodule?</p>