get rid of num_packed_objects()

The coming index format change doesn't allow for the number of objects
to be determined from the size of the index file directly.  Instead, Let's
initialize a field in the packed_git structure with the object count when
the index is validated since the count is always known at that point.

(based on commit 57059091fa)

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-count-objects.c

@@ -110,7 +110,7 @@ int cmd_count_objects(int ac, const char **av, const char *prefix)
 		for (p = packed_git; p; p = p->next) {
 			if (!p->pack_local)
 				continue;
-			packed += num_packed_objects(p);
+			packed += p->num_objects;
 		}
 		printf("count: %lu\n", loose);
 		printf("size: %lu\n", loose_size / 2);