title: Show git diff on file in staging area
body: <p>Is there a way I can see the changes that were made to a <code>file</code> after I have done <code>git add file</code>?</p>
  
  <p>That is, when I do: </p>
  
  <pre><code>git add file
  git diff file
  </code></pre>
  
  <p>no diff is shown. I guess there's a way to see the differences since the last commit but I don't know what that is.</p>