git-submodule.sh: accept verbose flag in cmd_update to be non-quiet

In a56771a668 (builtin/pull: respect verbosity settings in submodules,
2018-01-25), we made sure to pass on both quiet and verbose flag from
builtin/pull.c to the submodule shell script. However git-submodule doesn't
understand a verbose flag, which results in a bug when invoking

  git pull --recurse-submodules -v [...]

There are a few different approaches to fix this bug:

1) rewrite 'argv_push_verbosity' or its caller in builtin/pull.c to
   cap opt_verbosity at 0. Then 'argv_push_verbosity' would only add
   '-q' if any.

2) Have a flag in 'argv_push_verbosity' that specifies if we allow adding
  -q or -v (or both).

3) Add -v to git-submodule.sh and make it a no-op

(1) seems like a maintenance burden: What if we add code after
the submodule operations or move submodule operations higher up,
then we have altered the opt_verbosity setting further down the line
in builtin/pull.c.

(2) seems like it could work reasonably well without more regressions

(3) seems easiest to implement as well as actually is a feature with the
    last-one-wins rule of passing flags to Git commands.

Reported-by: Jochen Kühner
Signed-off-by: Stefan Beller <sbeller@google.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -516,6 +516,9 @@ cmd_update()
 		-q|--quiet)
 			GIT_QUIET=1
 			;;
+		-v)
+			GIT_QUIET=0
+			;;
 		--progress)
 			progress="--progress"
 			;;