title: git-diff: Only show changes staged for committing
body: <p>I want to see diff on files that I've added to commit (<code>git add something</code>). I can do this:</p>
  
  <pre><code>git diff HEAD `git status -s | grep ^M | cut -c 4-`
  </code></pre>
  
  <p>Is there a shorter way?</p>