diff-cache: fix handling of unmerged files.
diff-cache: fix handling of unmerged files.

We've always warned about them properly, but we would then do the
wrong thing if that filename existed in the tree we were comparing
against (we'd think the file has been deleted, because we forgot
about the unmerged cases).

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git