clone: factor out dir_exists() helper

Two parts of git-clone's setup logic check whether a directory exists, and they both call stat directly with the same scratch "struct stat" buffer. Let's pull that into a helper, which has a few advantages: - it makes the purpose of the stat calls more obvious - it makes it clear that we don't care about the information in "buf" remaining valid - if we later decide to make the check more robust (e.g., complaining about non-directories), we can do it in one place Note that we could just use file_exists() for this, which has identical code. But we specifically care about directories, so this future-proofs us against that function later getting more picky about seeing actual files. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>