patch 8.0.1459: cannot handle change of directory

Problem:    Cannot handle change of directory.
Solution:   Add the DirChanged autocommand event. (Andy Massimino,
            closes #888)  Avoid changing directory for 'autochdir' too often.

src/vim.h

@@ -1276,6 +1276,7 @@ enum auto_event
     EVENT_CMDWINLEAVE,		/* before leaving the cmdline window */
     EVENT_COLORSCHEME,		/* after loading a colorscheme */
     EVENT_COMPLETEDONE,		/* after finishing insert complete */
+    EVENT_DIRCHANGED,		/* after changing directory as a result of user cmd */
     EVENT_FILEAPPENDPOST,	/* after appending to a file */
     EVENT_FILEAPPENDPRE,	/* before appending to a file */
     EVENT_FILEAPPENDCMD,	/* append to a file using command */