use SWAP macro

Apply the semantic patch swap.cocci to convert hand-rolled swaps to use
the macro SWAP.  The resulting code is shorter and easier to read, the
object code is effectively unchanged.

The patch for object.c had to be hand-edited in order to preserve the
comment before the change; Coccinelle tried to eat it for some reason.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

object.c

@@ -104,9 +104,7 @@ struct object *lookup_object(const unsigned char *sha1)
 		 * that we do not need to walk the hash table the next
 		 * time we look for it.
 		 */
-		struct object *tmp = obj_hash[i];
-		obj_hash[i] = obj_hash[first];
-		obj_hash[first] = tmp;
+		SWAP(obj_hash[i], obj_hash[first]);
 	}
 	return obj;
 }