ancestor: add a way to remove ancestors of bases from a given set This and missingancestors can share state, which will turn out to be perfect for set discovery.

ancestor: add a way to add to bases of a missing ancestor object This will be useful for setdiscovery, since with that we incrementally add to our knowledge of common nodes.

ancestor: add a way to test whether a missing ancestor object has bases This is pretty trivial so there's no unit test coverage for it. This will be used by setdiscovery.

ancestor: remove now-unused missingancestors function Callers should use revlog.incrementalmissingrevs instead.

revlog: switch findmissing* methods to incrementalmissingrevs This will allow us to remove ancestor.missingancestors in an upcoming patch.

revlog: add a method to get missing revs incrementally This will turn out to be useful for discovery.