refactor(message): propagate highlight id instead of attrs

Problem:  Highlight group id is not propagated to the end of the message call
          stack, where ext_messages are emitted.
Solution: Refactor message functions to pass along highlight group id
          instead of attr id.

src/nvim/buffer.c

@@ -2902,7 +2902,7 @@ void buflist_list(exarg_T *eap)
                    buf == curbuf ? (int64_t)curwin->w_cursor.lnum : (int64_t)buflist_findlnum(buf));
     }
 
-    msg_outtrans(IObuff, 0);
+    msg_outtrans(IObuff, 0, false);
     line_breakcheck();
   }