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
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..