Up to now, the pure Python lazyancestors had been taking the parents
function in its constructor, whereas Rust-backed variants would have needed
a revlog index.
With this change, instantiation of all lazyancestors work uniformely with
an index. This requires only minor wrapping in test-ancestor.py.
It could be argued that this introduces a new duplication, and that
therefore, parentsfunc(index) should be provided in some utility module and
used both from revlog and ancestor modules.