title: Undo git update-index --skip-worktree
body: <p>A while ago I did this to ignore changes to a file tracked by git:</p>
  
  <pre><code>git update-index --skip-worktree &lt;file&gt;
  </code></pre>
  
  <p>Now I actually want to commit changes to that file to source. How do I undo the effects of <code>skip-worktree</code>?</p>