move conflicts to reuc when removing index entries

When removing files from the index, remove the associated conflicts and move them to the REUC.

I'm stumped as to how to add a test for this, though.  We can't, it seems, add conflicts to the index directory.  We could expose some methods that allow for that.

Or we could add some test repository that has conflicts in it...

Preferences?

/cc @yishaigalatzer