Add Rugged::Diff#merge!
Add Rugged::Diff#merge!

I named this "merge!" because the operation down in C is destructive
to the original diff the caller is merging the other diff onto.
We could probably have a "merge" one that returns a new Rugged::Diff
object but it makes memory management a little trickier.

Acidburn0zzz/rugged,libgit2/rugged,libgit2/rugged,saraid/rugged,tpickett66/rugged,tpickett66/rugged,tpickett66/rugged,Acidburn0zzz/rugged,libgit2/rugged,Acidburn0zzz/rugged,saraid/rugged,saraid/rugged