revlogutils: teach issue6528 filtering about grandparents During dynamic filtering, we should assume that the current repository is correct. Therefore the parents of the delta base can tell us if that parent has metadata without having to build the whole text.

revlogutils: remember known metadata parents for issue6528 In the cases where the parent revs tell us for sure that the parent has metadata, remember this fact to avoid content recomputations later.

revlogutils: for issue6528 fix, pre-cache nullrev as metadata-free