title: git clean -ndX doesn't remove directory
body: <p>With <code>.gitignore</code>:</p>
  
  <p><code>
  foo/
  </code></p>
  
  <p>and a git repository of:</p>
  
  <pre><code>./quux
  ./quux/foo
  ./quux/foo/bar
  ./quux/foo/bar/baz
  </code></pre>
  
  <p>As <code>foo</code> is ignored, git assumes working directory is clean:</p>
  
  <pre><code>% git status
  # On branch master
  nothing to commit, working directory clean
  </code></pre>
  
  <p>The <code>git clean -ndx</code> prints:</p>
  
  <pre><code>% git clean -ndx
  Would remove quux/
  </code></pre>
  
  <p>But <code>git clean -ndX</code> doesn't print anything. I would expect it to remove <code>quux/</code> as well.</p>
  
  <p>Is this a bug or a some gitignore feature I don’t get?
  If I add some file to <code>quux</code> and commit it. Than git clean will try to remove <code>quux/foo/</code> as expected.</p>
  
  <p>My git is pretty new: <code>git version 1.8.3.4</code>.</p>