Add the `RECURSE_SUBMODULES_ONLY` enum value to submodule.h. This enum
value will be used in a later patch to push to indicate that only
submodules should be pushed, while the superproject should remain
unpushed.
Signed-off-by: Brandon Williams <bmwill@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>