ancestor: filter out initial revisions lower than stoprev

ancestor: add test showing inconsistency between __iter__ and __contains__

ancestors: ensure a consistent order even in the "inclusive" case It seems odds to first issue the "source" revs and then the other ancestors. In addition, doing so can break the other contract of always issuing a child before its parent. We update the code to apply the same logic to all yielded revision. No tests break so we seem in the clear except where we explicitly test the order.