revsetlang: check arguments passed to ancestors() before optimizing to only() Future patches will add depth parameter to ancestors(), which isn't compatible with only().

revsetlang: factor out helper to match ancestors() in parsed tree More checks will be added.

templatekw: use common "rev:node" format as the default of predecessors I'm not sure if this is better. If we're planning to add a template keyword that returns obsoleted nodes unavailable in the repo (i.e. they have no valid revision numbers), we might want to use the current "node"-only format everywhere.

templatekw: populate all keywords depending on predecessor in map operation This is what showparents() does. repo[precnode] should never fail since its validity is tested by closestpredecessors().