builtin/submodule: allow cloning with different ref storage format

builtin/submodule: allow cloning with different ref storage format

As submodules are proper self-contained repositories, it is perfectly
valid for them to have a different ref storage format than their parent
repository. There is no obvious way for users to ask for the ref storage
format when initializing submodules though. Whether the setup of such
mixed-ref-storage-format constellations is all that useful remains to be
seen. But there is no good reason to not expose such an option, and we
will require it in a subsequent patch.

Introduce a new `--ref-format=` option for git-submodule(1) that allows
the user to pick the ref storage format. This option will also be used
in a subsequent commit, where we start to propagate the same flag from
git-clone(1) to cloning submodules with the `--recursive` switch.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>