builtin/clone: propagate ref storage format to submodules

builtin/clone: propagate ref storage format to submodules

When recursively cloning a repository with a non-default ref storage
format, e.g. by passing the `--ref-format=` option, then only the
top-level repository will end up using that ref storage format, and
all recursively cloned submodules will instead use the default format.

While mixed-format constellations are expected to work alright, the
outcome still is somewhat surprising as we have essentially ignored
the user's request.

Fix this by propagating the requested ref format to cloned submodules.

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