submodule update: silence underlying merge/rebase with "--quiet"

submodule update: silence underlying merge/rebase with "--quiet"

Commands such as

    $ git pull --rebase --recurse-submodules --quiet

produce non-quiet output from the merge or rebase.  Pass the --quiet
option down when invoking "rebase" and "merge".

Also fix the parsing of git submodule update -v.

When e84c3cf3 (git-submodule.sh: accept verbose flag in cmd_update
to be non-quiet, 2018-08-14) taught "git submodule update" to take
"--quiet", it apparently did not know how ${GIT_QUIET:+--quiet}
works, and reviewers seem to have missed that setting the variable
to "0", rather than unsetting it, still results in "--quiet" being
passed to underlying commands.

Signed-off-by: Theodore Dubois <tbodt@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>