TopoSort: Count unvisited ancestor instead of BitVector of 'visited' status

This is much more performant on the dir layer: it only takes 27 hours on
swh1.enst.fr instead of an ETA of one or two years, most of it spent on
this particular line:

```
LazyLongIterator successorAncestors = graph.successors(successorNodeId);
```

even when replacing all the code that used `successorAncestors`, it was
still the major cause of the huge expected runtime.