Autoload `split-string-shell-command'

* lisp/shell.el (split-string-shell-command): Autoload.

lisp/shell.el

@@ -459,6 +459,7 @@ Useful for shells like zsh that has this feature."
           (push (mapconcat #'identity (nreverse arg) "") args)))
       (cons (nreverse args) (nreverse begins)))))
 
+;;;###autoload
 (defun split-string-shell-command (string)
   "Split STRING (a shell command) into a list of strings.
 General shell syntax, like single and double quoting, as well as