Steps to reproduce: 

- Create a new wiki via ikiwiki --setup /etc/ikiwiki/auto.setup
- Test your wiki and the recentchanges page
- add the pandoc plugin to the setup file and set correct path to executable (e.g. `/usr/bin/pandoc`)
- run `ikiwiki --setup mywiki.setup` again

Then the recentchanges page doesn't work anymore.

Pandoc version: 1.19.2.1
Ikwiki version: 3.20170622

