submodule foreach: fix "<command> --quiet" not being respected

submodule foreach: fix "<command> --quiet" not being respected

Robin reported that

    git submodule foreach --quiet git pull --quiet origin

is not really quiet anymore [1]. "git pull" behaves as if --quiet is not
given.

This happens because parseopt in submodule--helper will try to parse
both --quiet options as if they are foreach's options, not git-pull's.
The parsed options are removed from the command line. So when we do
pull later, we execute just this

    git pull origin

When calling submodule helper, adding "--" in front of "git pull" will
stop parseopt for parsing options that do not really belong to
submodule--helper foreach.

PARSE_OPT_KEEP_UNKNOWN is removed as a safety measure. parseopt should
never see unknown options or something has gone wrong. There are also
a couple usage string update while I'm looking at them.

While at it, I also add "--" to other subcommands that pass "$@" to
submodule--helper. "$@" in these cases are paths and less likely to be
--something-like-this. But the point still stands, git-submodule has
parsed and classified what are options, what are paths. submodule--helper
should never consider paths passed by git-submodule to be options even
if they look like one.

The test case is also contributed by Robin.

[1] it should be quiet before fc1b9243cd (submodule: port submodule
    subcommand 'foreach' from shell to C, 2018-05-10) because parseopt
    can't accidentally eat options then.

Reported-by: Robin H. Johnson <robbat2@gentoo.org>
Tested-by: Robin H. Johnson <robbat2@gentoo.org>
Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>