In the example ikiwiki.setup:

    diffurl => "http://localhost:8000/?fd=\[[changeset]];file=\[[file]]",
    
should be:

    diffurl => "http://localhost:8000/?fd=\[[r2]];file=\[[file]]",

(changeset doesn't get expanded; r2 does, to what appears to be The Right Thing)

[[done]]
