submodule--helper: use OPT_SUBCOMMAND() API

submodule--helper: use OPT_SUBCOMMAND() API

Have the cmd_submodule__helper() use the OPT_SUBCOMMAND() API
introduced in fa83cc834da (parse-options: add support for parsing
subcommands, 2022-08-19).

This is only a marginal reduction in line count, but once we start
unifying this with a yet-to-be-added "builtin/submodule.c" it'll be
much easier to reason about those changes, as they'll both use
OPT_SUBCOMMAND().

We don't need to worry about "argv[0]" being NULL in the die() because
we'd have errored out in parse_options() as we're not using
"PARSE_OPT_SUBCOMMAND_OPTIONAL".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>