title: git rm -r doesn't work in a submodule
body: <p>Try to change submodule as described in <a href="http://git-scm.com/book/en/Git-Tools-Submodules" rel="nofollow">git book</a></p>
  
  <pre><code>$ git rm -r vendor/html-minifier
  $ git submodule add https://github.com/kangax/html-minifier.git vendor/html-minifier
  </code></pre>
  
  <p>But it fails with following</p>
  
  <pre><code>rm 'vendor/html-minifier'
  fatal: git rm: 'vendor/html-minifier': Is a directory
  </code></pre>