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