merge: don't overwrite file untracked after remove, abort with 'untracked files'

Merge could overwrite untracked files and cause data loss.

Instead we now handle the 'local side removed file and has untracked file
instead' case as the 'other side added file that local has untracked' case:

  FILE: untracked file exists
  abort: untracked files in working directory differ from files in requested revision

It could perhaps make sense to create .orig files when overwriting, either
instead of aborting or when overwriting anyway because of force ... but for now
we stay consistent with similar cases.

--HG--
branch : stable