refs: convert each_reflog_ent_fn to struct object_id

refs: convert each_reflog_ent_fn to struct object_id

Make each_reflog_ent_fn take two struct object_id pointers instead of
two pointers to unsigned char.  Convert the various callbacks to use
struct object_id as well.  Also, rename fsck_handle_reflog_sha1 to
fsck_handle_reflog_oid.

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