vim-patch:8.1.1757: text added with appendbufline() isn't displayed

Problem:    Text added with appendbufline() to another buffer isn't displayed.
Solution:   Update topline. (partly by Christian Brabandt, closes vim/vim#4718)
2984666291

src/nvim/eval.c

@@ -15194,6 +15194,7 @@ static void set_buffer_lines(buf_T *buf, linenr_T lnum_arg, bool append,
       }
     }
     check_cursor_col();
+    update_topline();
   }
 
   if (!is_curbuf) {