submodule: update and add must honor --quiet flag

submodule: update and add must honor --quiet flag

When using the --quiet flag "git submodule update" and "git submodule add"
didn't behave as the documentation stated. They printed progress output
from the clone, even though they should only print error messages.

Fix that by passing the -q flag to git clone in module_clone() when the
GIT_QUIET variable is set. Two tests in t7400 have been modified to test
that behavior.

Reported-by: Daniel Holtmann-Rice <flyingtabmow@gmail.com>
Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>