commit.c: rearrange xcalloc arguments

xcalloc() takes two arguments: the number of elements and their size.
reduce_heads() passes the arguments in reverse order, passing the
size of a commit*, followed by the number of commit* to be allocated.

Rearrange them so they are in the correct order.

Signed-off-by: Brian Gesiak <modocache@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -1031,7 +1031,7 @@ struct commit_list *reduce_heads(struct commit_list *heads)
 		p->item->object.flags |= STALE;
 		num_head++;
 	}
-	array = xcalloc(sizeof(*array), num_head);
+	array = xcalloc(num_head, sizeof(*array));
 	for (p = heads, i = 0; p; p = p->next) {
 		if (p->item->object.flags & STALE) {
 			array[i++] = p->item;