match: make unionmatcher a proper matcher unionmatcher is currently used where only a limited subset of its functions will be called. Specifically, visitdir() is never called. The next patch will pass it to dirstate.walk() where it will matter that visitdir() is correctly implemented, so let's fix that. Also add the explicitdir etc that will also be assumed by dirstate.walk() to exist on a matcher. Differential Revision: https://phab.mercurial-scm.org/D58