graft: test coverage of grafts and how merges can break duplicate detection This demonstrates unfortunate behaviour: extending the graft range cause the graft to behave differently. When the graft range includes a merge, we fail to detect duplicates that are ancestors of the merge.

mpatch: proxy through mdiff module See the previous commit for why.

bdiff: proxy through mdiff module See the previous commit for why. mdiff seems a good place to host bdiff functions. bdiff.bdiff was already aliased as textdiff, so we use it.