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/optiondefs.h b/src/optiondefs.h
index 585604f..81bb1db 100644
--- a/src/optiondefs.h
+++ b/src/optiondefs.h
@@ -1778,6 +1778,9 @@
     {"mousetime",   "mouset",	P_NUM|P_VI_DEF,
 			    (char_u *)&p_mouset, PV_NONE, NULL, NULL,
 			    {(char_u *)500L, (char_u *)0L} SCTX_INIT},
+    {"msghistory","mhi",    P_NUM|P_VI_DEF,
+			    (char_u *)&p_mhi, PV_NONE, NULL, NULL,
+			    {(char_u *)500L, (char_u *)0L} SCTX_INIT},
     {"mzquantum",  "mzq",   P_NUM,
 #ifdef FEAT_MZSCHEME
 			    (char_u *)&p_mzq, PV_NONE, did_set_mzquantum, NULL,