evolve: create resolved cset on successors of current parents
While resolving content-divergence, sometimes, the parent can be obsolete and
have a successor. We should consider the successors of the parents instead of
obsolete parents as that will do something like orphan+content-divergence
resolution both at the same time which is powerful and results in a single stack
while resolving content-divergence of stacks.
The test changes demonstrates that we can now successfully evolve a content
divergent stack.