match: add visitchildrenset complement to visitdir `visitdir(d)` lets a caller query whether the directory is part of the matcher. It can receive a response of 'all' (yes, and all children, you can stop calling visitdir now), False (no, and no children either), or True (yes, either something in this directory or a child is part of the matcher). `visitchildrenset(d)` augments that by instead of returning True, it returns a list of items to actually investigate. With this, code can be modified from: for f in self.all_items: if match.visitdir(self.dir + '/' + f): <do stuff> to be: for f in self.all_items.intersect(match.visitchildrenset(self.dir)): <do stuff> use of this function can provide significant performance improvements, especially when using narrow (so that the matcher is much smaller than the stuff we see on disk) and/or treemanifests (so that we can avoid loading manifests for trees that aren't part of the matcher). Differential Revision: https://phab.mercurial-scm.org/D4130