Automagically resolve conflicts upon remote deletion/moves

Scenario:

- Instructor creates repo with file ``A/b``
- Student nggitpulls repo
- Student edit ``A/b``
- Instructor ``git rm A/b`` (variant: ``git mv A/b A/c``)
- Student nbgitpulls latest version

This currently results in a merge conflict. Instead, in the general spirit of ``nbgitpuller`` some opinionated decision should be made.
Presumably:
- leave local file untouched (tracked by git? not any more?)
- warn the user: ``A/b was removed/renamed remotely; left local copy untouched``

(thanks btw for this useful tool!)