submodule--helper: rename "int res" to "int ret"

submodule--helper: rename "int res" to "int ret"

Rename the "res" variable added in b3c5f5cb048 (submodule: move core
cmd_update() logic to C, 2022-03-15) to "ret", which is the convention
in the rest of this file.

Eventual follow-up commits will change the code in update_submodule()
to a "goto cleanup" pattern, let's have the post image look consistent
with the rest. For update_submodules() let's also use a "ret" for
consistency, that use was also added in b3c5f5cb048. We'll be
modifying that codepath in subsequent commits.

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>