D5443: ancestor: uniformity of calling lazyancestors classes

[Yuya Nishihara]

> +def parentsfunc(index):
> +    def parentrevs(rev):
> +        try:
> +            entry = index[rev]
> +        except IndexError:
> +            if rev == wdirrev:
> +                raise error.WdirUnsupported
> +            raise
> +
> +        return entry[5], entry[6]
> +    return parentrevs

It doesn't seem correct that the ancestor module handles the revlog's
internals. Instead, can't we add a factory function which takes both
`index` and `pfunc`?