"Client asked me to extend back to the previous hop" in small networks

When chutney runs under tor's make test-network (bridges+hs flavour) on master, it sometimes logs a message:

Client asked me to extend back to the previous hop

This could be due to the new guard code, so I'm putting it in 0.3.0 until we know for sure. (I've not seen it in 0.2.9, but it's hard to know for sure, because it's intermittent.)