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/runtime/optwin.vim b/runtime/optwin.vim
index 99b1ecf..c8170da 100644
--- a/runtime/optwin.vim
+++ b/runtime/optwin.vim
@@ -749,6 +749,8 @@
 call <SID>BinOptionG("terse", &terse)
 call <SID>AddOption("shortmess", gettext("list of flags to make messages shorter"))
 call <SID>OptionG("shm", &shm)
+call <SID>AddOption("msghistory", gettext("how many messages are remembered"))
+call append("$", " \tset mhi=" . &mhi)
 call <SID>AddOption("showcmd", gettext("show (partial) command keys in location given by 'showcmdloc'"))
 let &sc = s:old_sc
 call <SID>BinOptionG("sc", &sc)