trac changeset browser won't give unified diff

[warner]

Greg Ward reports that our Trac's changeset browser gives back an empty file when you ask it for a unified diff (via the links at the bottom). For example, http://buildbot.net/trac/changeset/493 contains a link to http://buildbot.net/trac/changeset/493?format=diff&new=493 , but that link returns an empty body.

I suspect a bug in the trac+darcs plugin. Greg says this worked a few weeks ago. I don't know of anything that's changed in our Trac config since then, and I'm not seeing any error messages (nor progress messages) in the logs when I hit this link.