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.

test-ancestor: add support for multiple tests against one incremental object In upcoming patches we'll add more operations to the object, and this prepares for testing those operations.

test-ancestor: move naive missing ancestor algorithm into a class This mirrors the change to the real missing ancestor algorithm in a previous patch.

ancestor.missingancestors: turn into a state-keeping class This allows multiple efficient missing ancestor queries against the same set of bases. In upcoming patches we'll also define ways to grow the set of bases. The fact that the test output hasn't changed establishes this patch's correctness.

ancestor.missingancestors: calculate start point after filtering revsvisit Any revs that are filtered out are also in basesvisit, which means they wouldn't be returned in the missing list anyway. There's no need to explore such revs or their ancestors. The 'if not revsvisit' check moves down because we can't call max() on an empty set.