title: git clean not removing sub-directories (not recursive)
body: <p>I am facing problems with <code>git clean</code>.
  Consider the following scenario:</p>
  
  <pre><code>git status -su
  ?? file_1
  ?? xyz/file_2
  
  git clean -f
  Not removing xyz/file_2
  Removing file_1
  </code></pre>
  
  <p>I don't want to remove the <code>xyz</code> folder, but I want to remove the <code>file_2</code> inside it.</p>
  
  <p>Why is <code>git clean</code> is not working recursively?</p>