dir.c: avoid stat() in valid_cached_dir()

dir.c: avoid stat() in valid_cached_dir()

stat() may follow a symlink and return stat data of the link's target
instead of the link itself. We are concerned about the link itself.

It's kind of hard to demonstrate the bug. I think when path->buf is a
symlink, we most likely find that its target's stat data does not
match our cached one, which means we ignore the cache and fall back to
slow path.

This is performance issue, not correctness (though we could still
catch it by verifying test-dump-untracked-cache. The less unlikely
case is, link target stat data matches the cached version and we
incorrectly go fast path, ignoring real data on disk. A test for this
may involve manipulating stat data, which may be not portable.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>