also fetch used images when fetching 'redirected revisions'

when fetching by revision, and the revision points to a redirected
page, we already did fetch the redirected page by title. we also need
to call fetch_used for that title.

here's an example metabook.json that revealed the bug:

,----[ metabook.json ]
| {
|     "items": [
|         {
|             "content_type": "text/x-wiki",
|             "currentVersion": 1,
|             "latest": 423796393,
|             "revision": 423796393,
|             "timestamp": "1302664603",
|             "title": "ESRI grid",
|             "type": "article",
|             "url": "http://en.wikipedia.org/wiki/ESRI_grid"
|         }
|     ],
|     "licenses": [],
|     "subtitle": "",
|     "summary": "",
|     "title": "GMS 2",
|     "type": "collection",
|     "version": 1,
|     "wikis": [
|         {
|             "baseurl": "http://en.wikipedia.org/w/",
|             "imagesize": 1200,
| 	    "script_extension": ".php",
|             "type": "wikiconf"
|         }
|     ]
| }
`----