repository: support unabsorbed in repo_submodule_init
In preparation for a subsequent commit that migrates code using
add_submodule_odb() to repo_submodule_init(), teach
repo_submodule_init() to support submodules with unabsorbed gitdirs.
(See the documentation for "git submodule absorbgitdirs" for more
information about absorbed and unabsorbed gitdirs.)
Signed-off-by: Jonathan Tan <jonathantanmy@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>