submodule--helper: understand --checkout, --merge and --rebase synonyms

submodule--helper: understand --checkout, --merge and --rebase synonyms

Understand --checkout, --merge and --rebase synonyms for
--update={checkout,merge,rebase}, as well as the short options that
'git submodule' itself understands.

This removes a difference between the CLI API of "git submodule" and
"git submodule--helper", making it easier to make the latter an alias
for the former. See 48308681b07 (git submodule update: have a
dedicated helper for cloning, 2016-02-29) for the initial addition of
--update.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>