refs/reftable-backend: drop uses of the_repository

refs/reftable-backend: drop uses of the_repository

reftable_be_init() and reftable_be_create_on_disk() use the_repository even
though a repository instance is already available, either directly or via
struct ref_store.

Replace these uses with the appropriate local repository instance (repo or
ref_store->repo) to avoid relying on global state.

Note that USE_THE_REPOSITORY_VARIABLE cannot be removed yet, as
is_bare_repository() is still there in the file.

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