vim-patch:8.1.1498: ":write" increments b:changedtick even though nothing changed

Problem:    ":write" increments b:changedtick even though nothing changed.
            (Daniel Hahler)
Solution:   Only increment b:changedtick if the modified flag is reset.
c024b46678

src/nvim/undo.c

@@ -2294,7 +2294,7 @@ static void u_undoredo(int undo, bool do_buf_event)
   if (old_flags & UH_CHANGED) {
     changed();
   } else {
-    unchanged(curbuf, FALSE);
+    unchanged(curbuf, false, true);
   }
 
   // because the calls to changed()/unchanged() above will bump changedtick