rev-parse: fix --verify to error out when passed junk after a good rev

Before this patch something like: $ git rev-parse --verify <good-rev> <junk> worked whatever junk was as long as <good-rev> could be parsed correctly. This patch makes "git rev-parse --verify" error out when passed any junk after a good rev. Signed-off-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>