name-hash.c: replace cache_name_compare() with memcmp(3)

The same_name() private function wants a quick-and-exact check to
see if they two names are byte-for-byte identical first and then
fall back to the slow path.  Use memcmp(3) for the former to make it
clear that we do not want any "name" specific comparison.

Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>