init: use setup.c's repo version verification

init: use setup.c's repo version verification

We check our templates to make sure they are from a
version of git we understand (otherwise we would init a
repository we cannot ourselves run in!). But our simple
integer check has fallen behind the times. Let's use the
helpers that setup.c provides to do it right.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>