use total_seconds() when calculating search change diff

as tested like:

    $ python -c "from datetime import datetime; import time; a = datetime.now(); time.sleep(0.5); b = datetime.now(); print((b-a).seconds)"
    0
    $ python -c "from datetime import datetime; import time; a = datetime.now(); time.sleep(0.5); b = datetime.now(); print((b-a).total_seconds())"
    0.50064

sublime/cache_manager.py

@@ -1030,7 +1030,7 @@ class CacheManager(metaclass=Singleton):
             # This future actually does the search and calls the
             # search_callback when each of the futures completes.
             def do_search():
-                # Sleep far a little while before returning the local results.
+                # Sleep for a little while before returning the local results.
                 # They are less expensive to retrieve (but they still incur
                 # some overhead due to the GTK UI main loop queue).
                 sleep(0.2)