[PATCH 1 of 3 gca-revset V3] revsets: add commonancestors revset

[Yuya Nishihara]

On Mon, 09 Jul 2018 17:14:27 -0700, Sean Farley wrote:
> # HG changeset patch
> # User Sean Farley <sean at farley.io>
> # Date 1529376114 25200
> #      Mon Jun 18 19:41:54 2018 -0700
> # Branch gca-revset
> # Node ID 20b61392326c251c5898ab34a610829113e3da0b
> # Parent  c153f440682f6b0d512065a915c9f4909ed05180
> revsets: add commonancestors revset

Queued the series, thanks.

> + at predicate('commonancestors(set)', safe=True)
> +def commonancestors(repo, subset, x):

I've moved this functions to be sorted lexicographically.