match: teach diffmatcher.visitdir() to return 'all' if possible This patch teaches differencematcher.visitdir() to return 'all' when m1.visitdir() returns 'all' and m2 does not matches. Before this patch, from a differencematcher.visitdir(), we always returned either True or False. We never returned 'all' even when we can. This causes problem when m1 and m2 of a differencematcher are themselves differencematcher. In that case, we try to check: `if self._m2_.visitdir(dir) == 'all'` which will never be 'all' even though it can be. This leads to iterating over a lot of sub-directory manifest, even though we don't want to while extending a narrow clone. I am yet to measure the impact of this but calculating manifest was taking ~50-60 seconds, so this should definitely save some of time there. Differential Revision: https://phab.mercurial-scm.org/D5814

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