copies: refactor symmetricdifference as _findlimit We only need to track the lowest revision seen, which makes things simpler.

copies: teach symmetric difference about working revisions - use changelog.count() as a pseudo revision number - abort early in copies if revs are the same - eliminate working dir hacks in copies - yield results as they're found

symmetricdifference: move back to copies It's too tightly dependent on known revlog ordering to fit well in ancestors