submodule--helper update-clone: learn --init

submodule--helper update-clone: learn --init

Teach "git submodule--helper update-clone" the --init flag and remove
the corresponding shell code.

When the `--init` flag is passed to the subcommand, we do not spawn a
new subprocess and call `submodule--helper init` on the submodule paths,
because the Git machinery is not able to pick up the configuration
changes introduced by that init call. So we instead run the
`init_submodule_cb()` callback over each submodule in the same process.

[1] https://lore.kernel.org/git/CAP8UFD0NCQ5w_3GtT_xHr35i7h8BuLX4UcHNY6VHPGREmDVObA@mail.gmail.com/

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>