ignore forks when figuring date for sitemap

Fix for issue https://github.com/fedwiki/wiki-node/issues/20

Now Recent Changes will show pages with exactly the same last edit without spacing.
(This use to happen only when rsync'ing pages between sites.)

![image](https://cloud.githubusercontent.com/assets/12127/5430723/17e340de-83cc-11e4-8201-5c6c2d2788be.png)