submodule--helper: have --require-init imply --init

submodule--helper: have --require-init imply --init

Adjust code added in 0060fd1511b (clone --recurse-submodules: prevent
name squatting on Windows, 2019-09-12) to have the internal
--require-init option imply --init, rather than having
"git-submodule.sh" add it implicitly.

This change doesn't make any difference now, but eliminates another
special-case where "git submodule--helper update"'s behavior was
different from "git submodule update". This will make it easier to
eventually replace the cmd_update() function in git-submodule.sh.

We'll still need to keep the distinction between "--init" and
"--require-init" in git-submodule.sh. Once cmd_update() gets
re-implemented in C we'll be able to change variables and other code
related to that, but not yet.

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