This is in preparation for an upcoming refactoring. This also fixes a bug in
incancestors, where if an element of revs was an ancestor of another it would
be generated twice.
This will be used as a step in removing reachable() in a future diff.
Doing it now because bryano is in the process of rewriting ancestors in
C. This depends on bryano's patch to replace *revs with revs in the
declaration of revlog.ancestors.
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
