title: Redo merge of just a single file
body: <p>I'm in the middle of a large merge, and I've used <code>git mergetool</code> to resolve all the conflicts, but I have not committed yet, as I wanted to make sure the merge was ok first.</p>
  
  <p>It turns out that I made a mistake while resolving the conflicts in one file, and I would like to redo the conflict resolution with <code>git mergetool</code> on that file. As this is a large merge I would like to avoid redoing the merge on all the other files, as I understand I would have to do with <code>git merge --abort</code>.</p>
  
  <p>I know I could just edit the file manually, but this would be quite tedious and it would be much easier to just redo the <code>git mergetool</code> operation. Is this possible?</p>