org-agenda.el: Fixed a problem when computing time-grid.

Memnon Anon reported that this setting yields a bug:

,----
| (setq org-agenda-time-grid (quote
|       ((daily weekly today require-timed) "----------------"
|       ( 000 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2359))))
`----

lisp/org-agenda.el

@@ -5118,7 +5118,7 @@ The modified list may contain inherited tags, and tags matched by
 	  (throw 'exit list))
       (while (setq time (pop gridtimes))
 	(unless (and remove (member time have))
-	  (setq time (int-to-string time))
+	  (setq time (format "%2d" time))
 	  (push (org-format-agenda-item
 		 nil string "" nil
 		 (concat (substring time 0 -2) ":" (substring time -2)))