Fix PageAggregate::collectHierarchyGrandchildren().
Fix PageAggregate::collectHierarchyGrandchildren().

edlund/silverstripe-patchwork,redema/silverstripe-patchwork,edlund/silverstripe-patchwork,redema/silverstripe-patchwork,edlund/silverstripe-patchwork,redema/silverstripe-patchwork