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>

Documentation/git-rev-parse.txt

@@ -52,6 +52,11 @@ OPTIONS
 	The parameter given must be usable as a single, valid
 	object name.  Otherwise barf and abort.
 
+-q, --quiet::
+	Only meaningful in `--verify` mode. Do not output an error
+	message if the first argument is not a valid object name;
+	instead exit with non-zero status silently.
+
 --sq::
 	Usually the output is made one line per flag and
 	parameter.  This option makes output a single line,