title: Restart/undo conflict resolution in a single file
body: <p>In a larger git merge with several conflicting files, I incorrectly marked a file as resolved (using <code>git add FILE</code> after some editing)</p>
  
  <p>Now I'd like to undo my conflict resolution attempt and start over resolving that file.</p>
  
  <p>How can I do that?</p>