tests/test-abort-checkin.t

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

  $ cat > abortcommit.py <<EOF
  > from mercurial import error
  > def hook(**args):
  >     raise error.Abort(b"no commits allowed")
  > def reposetup(ui, repo):
  >     repo.ui.setconfig(b"hooks", b"pretxncommit.nocommits", hook)
  > EOF
  $ abspath=`pwd`/abortcommit.py

  $ cat <<EOF >> $HGRCPATH
  > [extensions]
  > mq =
  > abortcommit = $abspath
  > EOF

  $ hg init foo
  $ cd foo
  $ echo foo > foo
  $ hg add foo

mq may keep a reference to the repository so __del__ will not be
called and .hg/journal.dirstate will not be deleted:

  $ hg ci -m foo
  error: pretxncommit.nocommits hook failed: no commits allowed
  transaction abort!
  rollback completed
  abort: no commits allowed
  [255]
  $ hg ci -m foo
  error: pretxncommit.nocommits hook failed: no commits allowed
  transaction abort!
  rollback completed
  abort: no commits allowed
  [255]

  $ cd ..