submodule--helper: accept '-i' shorthand for update --init

submodule--helper: accept '-i' shorthand for update --init

commit 3ad0ba722744 ("git-submodule.sh: improve variables readability")
made `git submodules update -i` pass `-i` as is to submodule--helper,
but it fails with `error: unknown switch `i'` because the helper does
not accept the short option.

All other short options supported by git-submodule.sh are properly
handle in the helper, so also add the alias for --init

Fixes: 3ad0ba722744 ("git-submodule.sh: improve variables readability")
Signed-off-by: Dominique Martinet <dominique.martinet@atmark-techno.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>