pack-objects: move thread autodetection closer to relevant code

Let's keep thread stuff close together if possible.  And in this case,
this even reduces the #ifdef noise, and allows for skipping the
autodetection altogether if delta search is not needed (like with a pure
clone).
Signed-off-by: NNicolas Pitre <nico@fluxnic.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-pack-objects.c

@@ -1629,6 +1629,8 @@ static void ll_find_deltas(struct object_entry **list, unsigned list_size,
 	struct thread_params *p;
 	int i, ret, active_threads = 0;
 
+	if (!delta_search_threads)	/* --threads=0 means autodetect */
+		delta_search_threads = online_cpus();
 	if (delta_search_threads <= 1) {
 		find_deltas(list, &list_size, window, depth, processed);
 		return;
@@ -2324,11 +2326,6 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 	if (keep_unreachable && unpack_unreachable)
 		die("--keep-unreachable and --unpack-unreachable are incompatible.");
 
-#ifdef THREADED_DELTA_SEARCH
-	if (!delta_search_threads)	/* --threads=0 means autodetect */
-		delta_search_threads = online_cpus();
-#endif
-
 	prepare_packed_git();
 
 	if (progress)