D4327: dagop: extract headsetofconnecteds() from dagutil

[Yuya Nishihara]

> +def headrevs(revs, parentsfn):
> +    """Resolve the set of heads from a set of revisions.
> +
> +    Receives an iterable of revision numbers and a callbable that receives a
> +    revision number and returns an iterable of parent revision numbers, possibly
> +    including nullrev.
> +
> +    Returns a set of revision numbers that are DAG heads within the passed subset.
> +    """
> +    headrevs = set()

Absorbed fix from D4328 since this never be non-empty set.

Overall, the series looks really nice. Thanks.