Rename suffixcmp() to has_suffix() and invert its result

Now has_suffix() returns 1 when the suffix is present and 0 otherwise.

The old name followed the pattern anything-cmp(), which suggests
a general comparison function suitable for e.g. sorting objects.
But this was not the case for suffixcmp().

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>