vim-patch:9.1.1871: Wrong 'showcmd' in the cmdwin (#36320)

Problem:  Wrong 'showcmd' in the command-line window.
Solution: Clear 'showcmd' when entering cmdwin (zeertzjq).

related: neovim/neovim#36271
closes: vim/vim#18616

826326f13b

src/nvim/ex_getln.c

@@ -4651,6 +4651,7 @@ static int open_cmdwin(void)
 
   State = MODE_NORMAL;
   setmouse();
+  clear_showcmd();
 
   // Reset here so it can be set by a CmdwinEnter autocommand.
   cmdwin_result = 0;