mergestate._resolve: handle change/delete conflicts We will represent a deleted file as 'nullhex' in the in-memory and on-disk merge states. We need to be able to create absentfilectxes in that case, and delete the file from disk rather than try to write it out.

filemerge: don't try to copy files known to be absent We set 'back' to None in this case, so we need to handle that as well.