largefiles: getstandinmatcher should not depend on existence of directories
Looking for a (potentially empty) directory was not reliable - both because it
is a reasonable assumption that empty directories can be removed and because it
wasn't created in all cases ... such as when pulling to an existing repository.
tests: don't rely on broken behaviour in test-largefiles-cache.t
The test relied on the bug that 'pull largefiles from branchheads' didn't pull
any largefiles from tip revision when it seemed like no largefiles had been
checked out before.