Resolve other kinds of conflicts

[smashwilson]

Conflict markers are modify-modify conflicts, indicated by U U in git status --porcelain, but there are other conflicts that are possible: modify and delete, both added, and so on.

Resolution is usually simpler for these cases, but (as I've hinted with the "modified by both") I'd like to display them in the MergeConflictsView and let you resolve them different ways with a context menu.