submodule--helper update-clone: check for --filter and --init

submodule--helper update-clone: check for --filter and --init

"git submodule update --filter" also requires the "--init" option. Teach
update-clone to do this usage check in C and remove the check from
git-submodule.sh.

In addition, change update-clone's usage string so that it teaches users
about "git submodule update" instead of "git submodule--helper
update-clone" (the string is copied from git-submodule.sh). This should
be more helpful to users since they don't invoke update-clone directly.

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>