typing: use python3-style type annotation

merge: cache the fs checks made during [_checkunknownfiles] this ~halves the number of lstat calls made when updating from rev(-1) to a revision with lots of files

merge: disable the whole filesystem access loop if [_realfs] is false This makes it clearer that [auditeddir] is only relevant for [_realfs] checkers, and makes the non-realfs checkers more performant.