Stgit - gitmergeonefile.py: handle removal vs. changes

I just got a "removal vs. changed" conflict, which is unhandled by StGit. That
is taken from git-merge-one-file resolver, but is bad, as stg resolved does not
handle unmerged entries (and probably it should be fixed too).

Sample patch included, but some thought must be done on it (see the comments I
left in).

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>