patch 8.0.0735: no indication that the quickfix window/buffer changed

Problem:    There is no way to notice that the quickfix window contents has
            changed.
Solution:   Increment b:changedtick when updating the quickfix window.
            (Yegappan Lakshmanan)

src/quickfix.c

@@ -3286,6 +3286,7 @@ qf_update_buffer(qf_info_T *qi, qfline_T *old_last)
 	qf_update_win_titlevar(qi);
 
 	qf_fill_buffer(qi, buf, old_last);
+	++CHANGEDTICK(buf);
 
 	if (old_last == NULL)
 	{