rev-parse --verify: do not output anything on error

rev-parse --verify: do not output anything on error

Before this patch, when "git rev-parse --verify" was passed at least one
good rev and then anything, it would output something for the good rev
even if it would latter exit on error.

With this patch, we only output something if everything is ok.

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