submodule update: silence underlying fetch with "--quiet"

Commands such as

    $ git submodule update --quiet --init --depth=1

involving shallow clones, call the shell function fetch_in_submodule, which
in turn invokes git fetch.  Pass the --quiet option onward there.

Signed-off-by: Nicholas Clark <nick@ccl4.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -420,9 +420,9 @@ fetch_in_submodule () (
 	cd "$1" &&
 	if test $# -eq 3
 	then
-		echo "$3" | git fetch --stdin ${2:+"$2"}
+		echo "$3" | git fetch ${GIT_QUIET:+--quiet} --stdin ${2:+"$2"}
 	else
-		git fetch ${2:+"$2"}
+		git fetch ${GIT_QUIET:+--quiet} ${2:+"$2"}
 	fi
 )