vim-patch:9.1.0798: too many strlen() calls in cmdhist.c (#30895)

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

closes: vim/vim#15888

8df07d0ca3

Co-authored-by: John Marriott <basilisk@internode.on.net>

src/nvim/cmdhist.h

@@ -23,6 +23,7 @@ enum { HIST_COUNT = HIST_DEBUG + 1, };  ///< Number of history tables
 typedef struct {
   int hisnum;           ///< Entry identifier number.
   char *hisstr;         ///< Actual entry, separator char after the NUL.
+  size_t hisstrlen;     ///< Length of hisstr (excluding the NUL).
   Timestamp timestamp;  ///< Time when entry was added.
   AdditionalData *additional_data;  ///< Additional entries from ShaDa file.
 } histentry_T;