rebase: allow rebasing children of wd to wd if a new branch has been set (BC) The named branch of the leaf changeset can be changed by updating to it, setting the branch, and amending. But previously, there was no good way to *just* change the branch of several linear changes. If rebasing changes with another parent to '.', it would pick up a pending branch change up. But when rebasing changes that have the same parent, it would fail with 'nothing to rebase', even when the branch name was set differently. To fix this, allow rebasing to same parent when a branch has been set.