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>