Fix M-x hints on Mac port

* lisp/simple.el (execute-extended-command--shorter): Call
input-pending-p to trigger input processing on some systems, such
as Mac port.  (Bug#23002)

lisp/simple.el

@@ -1663,6 +1663,7 @@ If the value is non-nil and not a number, we wait 2 seconds."
                   ;; Don't show the help message if the binding isn't
                   ;; significantly shorter than the M-x command the user typed.
                   (< len (- max 5))))
+      (input-pending-p)    ;Dummy call to trigger input-processing, bug#23002.
       (let ((candidate (pop candidates)))
         (when (equal name
                        (car-safe (completion-try-completion