Add API to determine undo status

This is is useful to determine if an undoable action was
performed before adding additional information (e.g. selection
changes) to the undo transaction.

libs/ardour/ardour/session.h

@@ -1075,6 +1075,28 @@ public:
 
 	bool operation_in_progress (GQuark) const;
 
+	/**
+	 * Test if any undo commands were added since the
+	 * call to begin_reversible_command ()
+	 *
+	 * This is is useful to determine if an undoable
+	 * action was performed before adding additional
+	 * information (e.g. selection changes) to the
+	 * undo transaction.
+	 *
+	 * @return true if undo operation is valid but empty
+	 */
+	bool collected_undo_commands () const {
+		return _current_trans && !_current_trans->empty ();
+	}
+
+	/**
+	 * Abort reversible commend IFF no undo changes
+	 * have been collected.
+	 * @return true if undo operation was aborted.
+	 */
+	bool abort_empty_reversible_command ();
+
 	void add_commands (std::vector<Command*> const & cmds);
 
 	std::map<PBD::ID,PBD::StatefulDestructible*> registry;

libs/ardour/luabindings.cc

@@ -2536,6 +2536,8 @@ LuaBindings::common (lua_State* L)
 		.addFunction ("begin_reversible_command", (void (Session::*)(const std::string&))&Session::begin_reversible_command)
 		.addFunction ("commit_reversible_command", &Session::commit_reversible_command)
 		.addFunction ("abort_reversible_command", &Session::abort_reversible_command)
+		.addFunction ("collected_undo_commands", &Session::collected_undo_commands)
+		.addFunction ("abort_empty_reversible_command", &Session::abort_empty_reversible_command)
 		.addFunction ("add_command", &Session::add_command)
 		.addFunction ("add_stateful_diff_command", &Session::add_stateful_diff_command)
 		.addFunction ("playlists", &Session::playlists)

libs/ardour/session_state.cc

@@ -3093,6 +3093,16 @@ Session::abort_reversible_command ()
 	}
 }
 
+bool
+Session::abort_empty_reversible_command ()
+{
+	if (!collected_undo_commands ()) {
+		abort_reversible_command ();
+		return true;
+	}
+	return false;
+}
+
 void
 Session::commit_reversible_command (Command *cmd)
 {