Ref T9948. Ref T13546. This change moves toward a functional "arc land" in Mercurial.

Because of how "bundlerepo.getremotechanges()" works, "hg arc-ls-markers" does not actually list markers in the remote that aren't different from local markers so it's hard to get anywhere with this.

Got somewhat-encouraging output from "arc land" and "hg arc-ls-markers", but too many things are still broken for this to really work yet.