Add ancestors and descendants to revlog
This patch adds two methods to revlog:
- ancestors: given a list of revisions returns their ancestors
- descendants: given a list of revisions return their descendants
Ancestors of 5
4 2 0
Ancestors of 6 and 5
3 4 2 1 0
Ancestors of 5 and 4
4 2 0
Descendants of 5
7 8
Descendants of 5 and 3
6 7 8
Descendants of 5 and 4
5 7 8