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

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>