evolve: abort if deciding resolution parent is ambiguous While solving content-divergence, it could be hard to decide which cset should be treated as a resolution parent if there are multiple successors of divergent cset. i.e split case. But we are planning to make some update in handling this split case: 1) consider highest one if all splitted csets are in a single topological branch 2) still need to decide if 1) doesn't apply