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

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>