Draft: evolve: look for split successors of the correct ancestor (issue6648)

Consider two changesets, 1 and 2. 1 is split into two new changesets and 2 is pruned. If we stand on 2 and call hg evolve, _singlesuccessor() will traverse ancestors of wdp in search of a changeset with successors to update to (it will be 1, which was split). In case of a split, select_split_successor() gets control. The issue is this function didn't traverse ancestors, and instead tried to look up successors of the original changeset (i.e. 2 in our case, which was pruned).

We can make select_split_successor() aware of _singlesuccessor() logic by using the changeset that actually has successors without traversing ancestors again. It's done by storing that changeset in MultipleSuccessorsError exception.