evolve: consider all obsolete ancestors when finding new parent for orphan
Both the old and the new logic have in common that the orphan changeset is
moved to the tipmost successor of one or more ancestors of the orphan
changeset. The previous logic considered only the first non-pruned ancestor of
the orphan changeset. The new logic considers all obsolete ancestors of the
orphan changeset.
The logic in _possibledestination() had to be changed to be consistent with the
new logic.