Add "arc land" as a first-class workflow

Summary:
This is a fancy version of "land.sh" that uses "git merge --squash" and
"arc which" to cover more cases.

Test Plan:
Ran "arc land" against various repository states (no such branch, not
accepted, valid, etc). Things seemed OK. There are basically an infinite number
of states here so it's hard to test exhaustively.

Reviewers: cpiro, btrahan, jungejason, davidreuss

Reviewed By: davidreuss

CC: zeeg, aran, epriestley, davidreuss

Maniphest Tasks: T787, T723

Differential Revision: https://secure.phabricator.com/D1488