patch 9.1.0864: message history is fixed to 200

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

closes: #16048

Co-authored-by: Milly <milly.ca@gmail.com>
Signed-off-by: Shougo Matsushita <Shougo.Matsu@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/message.c b/src/message.c
index 576d922..5853e47 100644
--- a/src/message.c
+++ b/src/message.c
@@ -1013,7 +1013,7 @@
 	return;
 
     // Don't let the message history get too big
-    while (msg_hist_len > MAX_MSG_HIST_LEN)
+    while (msg_hist_len > p_mhi)
 	(void)delete_first_msg();
 
     // allocate an entry and add the message at the end of the history