prune: update to the successor of wdir also with --pair/--biject (issue6142)
When prune is used with --pair flag, we can also update to the successor of working directory parent. No need to check len(sucs) or len(precs) here because there's a check for that earlier in the code (it's a requirement of biject). The tests are now demonstrate the correct behavior: when rev 14 was pruned with 12 as its successor, the bookmark that was on 14 was moved to 12. That bookmark was also activated (even before this patch).