pack-objects: no delta possible with only one object in the list

... so don't even try in that case, and save another useless line of
progress display.
Signed-off-by: NNicolas Pitre <nico@cam.org>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

builtin-pack-objects.c

@@ -1714,7 +1714,7 @@ static void prepare_pack(int window, int depth)
 		delta_list[n++] = entry;
 	}
 
-	if (nr_deltas) {
+	if (nr_deltas && n > 1) {
 		unsigned nr_done = 0;
 		if (progress)
 			start_progress(&progress_state, "Deltifying objects",