template: add tests for more complex cases We add new tests for improving the coverage of existing obs-markers related template (predecessors) and the new one we are introducing (successorssets).

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.