Format "arc land" passthru commands more nicely, and execute them from CWD
Fixes T13380. Ref T13546. Use slightly nicer command formatting for passthru commands, to make it more obvious what's going on and which pieces of output are from "arc" vs various subcommands.
Also, execute repository API passthru commands from the working copy root. All other commands already did this, the older API just didn't support it.
Test Plan: Ran "arc land" in Git and Mercurial repositories, saw nicer output formatting.
Differential Revision: https://secure.phabricator.com/D21330