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>