refs/files-backend: convert static functions to object_id

refs/files-backend: convert static functions to object_id

Convert several static functions to take pointers to struct object_id.
Change the relevant parameters to write_packed_entry to be const, as we
don't modify them.  Rename lock_ref_sha1_basic to lock_ref_oid_basic to
reflect its new argument.  Update the docstring for verify lock to
account for the new parameter name, and note additionally that the
old_oid may be NULL.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>