largefiles: fall back to the original for change/delete conflicts The largefiles merge code (currently) does not handle change/delete conflicts. So fall back to regular filemerge in that case. Making this code handle change/delete conflicts is left as an exercise for the future.

mergestate.add: store absentfilectxes as nullhex This is the most natural way to represent these files. We also need to make sure we don't try to store these files in the merge store.