api: allow nvim_buf_attach from lua using callbacks

src/nvim/undo.c

@@ -2299,7 +2299,7 @@ static void u_undoredo(int undo, bool do_buf_event)
   // because the calls to changed()/unchanged() above will bump changedtick
   // again, we need to send a nvim_buf_lines_event with just the new value of
   // b:changedtick
-  if (do_buf_event && kv_size(curbuf->update_channels)) {
+  if (do_buf_event) {
     buf_updates_changedtick(curbuf);
   }