clone: factor out dir_exists() helper

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>