submodule--helper: return "ret", not "1" from update_submodule()

Amend the update_submodule() function to return the failing "ret" on error, instead of overriding it with "1". This code was added in b3c5f5cb048 (submodule: move core cmd_update() logic to C, 2022-03-15), and this change ends up not making a difference as this function is only called in update_submodules(). If we return non-zero here we'll always in turn return "1" in module_update(). But if we didn't do that and returned any other non-zero exit code in update_submodules() we'd fail the test that's being amended here. We're still testing the status quo here. This change makes subsequent refactoring of update_submodule() easier, as we'll no longer need to worry about clobbering the "ret" we get from the run_command(). Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Reviewed-by: Glen Choo <chooglen@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>