title: Git diff says subproject is dirty
body: <p>I have just run a git diff, and I am getting the following output for all of my approx 10 submodules </p>
  
  <pre><code>diff --git a/.vim/bundle/bufexplorer b/.vim/bundle/bufexplorer
  --- a/.vim/bundle/bufexplorer
  +++ b/.vim/bundle/bufexplorer
  @@ -1 +1 @@
  -Subproject commit 8c75e65b647238febd0257658b150f717a136359
  +Subproject commit 8c75e65b647238febd0257658b150f717a136359-dirty
  </code></pre>
  
  <p>What does this mean? How do I fix it?</p>