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>

refs.h

@@ -292,7 +292,7 @@ int delete_reflog(const char *refname);
 
 /* iterate over reflog entries */
 typedef int each_reflog_ent_fn(
-		unsigned char *old_sha1, unsigned char *new_sha1,
+		struct object_id *old_oid, struct object_id *new_oid,
 		const char *committer, unsigned long timestamp,
 		int tz, const char *msg, void *cb_data);