vim-patch:9.1.0864: message history is fixed to 200 (#31215)

Problem:  message history is fixed to 200
Solution: Add the 'msghistory' option, increase the default
          value to 500 (Shougo Matsushita)

closes: vim/vim#16048

4bd9b2b246

Co-authored-by: Shougo Matsushita <Shougo.Matsu@gmail.com>
Co-authored-by: Milly <milly.ca@gmail.com>

src/nvim/message.c

@@ -981,7 +981,7 @@ static void add_msg_hist_multihl(const char *s, int len, int hl_id, bool multili
   }
 
   // Don't let the message history get too big
-  while (msg_hist_len > MAX_MSG_HIST_LEN) {
+  while (msg_hist_len > p_mhi) {
     delete_first_msg();
   }