Fix bug in tile indexer saying that all times were new

tile_gen/tile_index.py

@@ -110,7 +110,7 @@ def index(input_files, output_shapefile):
     shutil.rmtree(tmp_dir, ignore_errors=True)
 
     times_removed = current_times - new_times
-    times_added = new_times = current_times
+    times_added = new_times - current_times
     return sorted(times_removed), sorted(times_added)