patch 9.1.0798: too many strlen() calls in cmdhist.c

Problem:  too many strlen() calls in cmdhist.c
Solution: refactor code and remove strlen() calls
          (John Marriott)

closes: #15888

Signed-off-by: John Marriott <basilisk@internode.on.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/structs.h

@@ -1283,6 +1283,7 @@ typedef struct hist_entry
     int		hisnum;		// identifying number
     int		viminfo;	// when TRUE hisstr comes from viminfo
     char_u	*hisstr;	// actual entry, separator char after the NUL
+    size_t	hisstrlen;	// length of hisstr (excluding the NUL)
     time_t	time_set;	// when it was typed, zero if unknown
 } histentry_T;