Fix automation write-mode (overwrite existing automation)

AutomationWatch::transport_state_change() is called at a time
when transport has just started to roll, but may not yet be rolling
(count-in, latency-preroll).

Later, after pre-roll is complete and transport starts moving,
AutomationWatch::transport_state_change() is not called again.

Thus set_in_write_pass(true); is never called, and writing
automation assumes that all changes happen while stopped.

In 'write" mode, guard points are added for each change, when
the transport is stopped.

Before this change, this happened at periodic intervals even
when rolling.

libs/ardour/automation_watch.cc

@@ -225,7 +225,7 @@ AutomationWatch::transport_state_change ()
 		return;
 	}
 
-	bool rolling = _session->transport_rolling();
+	bool rolling = _session->transport_state_rolling ();
 
 	_last_time = _session->audible_sample ();