submodule: accept -v for the update command

Since a56771a6 (builtin/pull: respect verbosity settings in
submodules, 2018-01-25), "git pull -v --recurse-submodules"
propagates the "-v" to the submodule command, but because the
latter command does not understand the option, it barfs.

Teach "git submodule update" to accept the option to fix it.

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -244,6 +244,9 @@ cmd_update()
 		-q|--quiet)
 			quiet=1
 			;;
+		-v|--verbose)
+			quiet=0
+			;;
 		--progress)
 			progress=1
 			;;