manifestmerge: have manifest do matching before diffing This means that the diff code does less work, potentially significantly less in the case of treemanifests. It also should ease implementation with narrowed clone cases (such as narrowhg) when we don't always have the entire set of treemanifest revlogs locally. As far as I can tell, this codepath is currently only used by record, so it'll probably die in the near future, and then narrowhg won't have to worry about composing with some unknown matching system.