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>

connect.c

@@ -254,7 +254,7 @@ static int process_dummy_ref(const struct packet_reader *reader)
 		return 0;
 	name++;
 
-	return oideq(&null_oid, &oid) && !strcmp(name, "capabilities^{}");
+	return oideq(null_oid(), &oid) && !strcmp(name, "capabilities^{}");
 }
 
 static void check_no_capabilities(const char *line, int len)