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/option.h b/src/option.h
index d5f72a1..18a7c2a 100644
--- a/src/option.h
+++ b/src/option.h
@@ -794,6 +794,7 @@
 #endif
 EXTERN long	p_mouset;	// 'mousetime'
 EXTERN int	p_more;		// 'more'
+EXTERN long	p_mhi;		// 'msghistory'
 #ifdef FEAT_MZSCHEME
 EXTERN long	p_mzq;		// 'mzquantum
 # if defined(DYNAMIC_MZSCHEME)