match: document that visitchildrenset might return files At least when using includematcher, and probably most matchers, we do not know if a/b/f refers to a file 'f' in a/b, or a subdirectory 'f' in a/b, so most matchers will return {'f'} for visitchildrenset('a/b'). Arguably, all matchers could/should - for exactmatcher, we know that 'f' is a file, but there's no reason to return 'this' for visitchildrenset('a/b') causing code to investigate 'a/b/x', for example. Differential Revision: https://phab.mercurial-scm.org/D4364