title: GIt merge doesn't remove files deleted by them
body: <p>I'm merging a branch into my own branch.
  the other branch ('them') removed a file I have edited.
  when I merge, I get a conflict. <code>git status</code> shows my file has been 'deleted by them'.</p>
  <p>when I add this deleted file and commit, the file still stays.
  Why is this? I want the file to be deleted, which is why I added its deletion.
  <a href="https://i.stack.imgur.com/aQedN.png" rel="nofollow noreferrer">Example screenshot here</a></p>