obsolete: closest divergent support
Add a closest argument to successorssets changing the definition of latest
successors.
With "closest=false" (current behavior), latest successors are "leafs" on the
obsmarker graph. They don't have any successor and are known locally.
With "closest=true", latest successors are the closest locally-known
changesets that are visible in the repository or repoview. Closest successors
can be then obsolete, orphan.
This will be used in a later patch to show the closest successor of
changesets with the successorssets template.