dim: add cat-to-fixup

This is useful for the very rare cases where git rerere fails and
can't remember a conflict resolution. Or the rare cases with silent
conflicts.

Since this happens so seldom I always forget the precise patch file
naming convention and waste half an hour debugging dim. Help the
brainless a bit out here.

Also s/git apply/ patch/ since git apply is too picky about unresolved
merges (which occasionally also need to be fixed up).