D4738: revset: make commonancestors(x + x^) be ::(x^), not ::x

[Yuya Nishihara]

Queued the series for stable, many thanks.

> -    # only wants the heads of the set passed in
> -    h = heads(repo, fullreposet(repo), x, anyorder)
> -    if not h:
> +    startrevs = getset(repo, fullreposet(repo), x)

I've resurrected the `anyorder` flag though that wouldn't matter much in
practice.