prune: replace --biject with --pair People have been complaining about `--biject` being to obscure for a while. A recent discussion with Pulkit Goyal and Dan Villiom Podlaski Christiansen produce some new concrete proposal that we liked. We eventually settle on using `--pair`. The `--biject` flag will be kept as an alias for a while.

topic: check that repo.currenttopic is set before using it as an argument repo.currenttopic can be an empty string, which means there's no current topic set. This fact can be used to short-circuit some checks. Also, this avoids passing an empty string as an argument to topic() revset.