hash: provide per-algorithm null OIDs

Up until recently, object IDs did not have an algorithm member, only a
hash.  Consequently, it was possible to share one null (all-zeros)
object ID among all hash algorithms.  Now that we're going to be
handling objects from multiple hash algorithms, it's important to make
sure that all object IDs have a correct algorithm field.

Introduce a per-algorithm null OID, and add it to struct hash_algo.
Introduce a wrapper function as well, and use it everywhere we used to
use the null_oid constant.

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

notes.c

@@ -1327,7 +1327,7 @@ int copy_note(struct notes_tree *t,
 	if (note)
 		return add_note(t, to_obj, note, combine_notes);
 	else if (existing_note)
-		return add_note(t, to_obj, &null_oid, combine_notes);
+		return add_note(t, to_obj, null_oid(), combine_notes);
 
 	return 0;
 }