refs: remove the_hash_algo global state

refs: remove the_hash_algo global state

refs.c uses the_hash_algo in multiple places, relying on global state for
the object hash algorithm. Replace these uses with the appropriate
repository-specific hash_algo. In transaction-related functions
(ref_transaction_create, ref_transaction_delete, migrate_one_ref, and
transaction_hook_feed_stdin), use transaction->ref_store->repo->hash_algo.
In other cases, such as repo_get_submodule_ref_store(), use
repo->hash_algo.

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