rev-parse: teach "--verify" to be quiet when using "-q" or "--quiet"

rev-parse: teach "--verify" to be quiet when using "-q" or "--quiet"

Currently "git rev-parse --verify <something>" is often used with
its error output redirected to /dev/null. This patch makes it
easier to do that.

The -q|--quiet option is designed to work the same way as it does
for "git symbolic-ref".

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