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>

builtin/diff.c

@@ -98,7 +98,7 @@ static int builtin_diff_b_f(struct rev_info *revs,
 
 	stuff_change(&revs->diffopt,
 		     blob[0]->mode, canon_mode(st.st_mode),
-		     &blob[0]->item->oid, &null_oid,
+		     &blob[0]->item->oid, null_oid(),
 		     1, 0,
 		     blob[0]->path ? blob[0]->path : path,
 		     path);