submodule--helper update: don't override 'checkout' exit code

submodule--helper update: don't override 'checkout' exit code

When "git submodule update" runs it might call "checkout", "merge",
"rebase", or a custom command. Ever since run_update_command() was
added in c51f8f94e5b (submodule--helper: run update procedures from C,
2021-08-24) we'd either exit immediately if the
"submodule.<name>.update" method failed, or in the case of "checkout"
continue trying to update other submodules.

This code used to use the magical "2" return code, but in
55b3f12cb54 (submodule update: use die_message(), 2022-03-15) it was
made to exit(128), which in preceding commits has been changed to
return that 128 code to the top-level.

Let's "libify" this code even more by not having it arbitrarily
override the return code. In practice this doesn't change anything as
the code "git checkout" would return on any normal failure is "1", but
we'll now in principle properly abort the operation if "git checkout"
were to exit with 128.

It would make sense to follow-up this change with a change to allow
the "submodule.<name>.update = !..." (SM_UPDATE_COMMAND) method the
same liberties as "checkout", and perhaps to do the same with a failed
"merge" or "rebase". But let's leave that for now.

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>