Make index preloading check the whole path to the file

Make index preloading check the whole path to the file

This uses the new thread-safe 'threaded_has_symlink_leading_path()'
function to efficiently verify that the whole path leading up to the
filename is a proper path, and does not contain symlinks.

This makes 'ce_uptodate()' a much stronger guarantee: it no longer just
guarantees that the 'lstat()' of the path would match, it also means
that we know that people haven't played games with moving directories
around and covered it up with symlinks.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>