message: revert arc immutability
  Summary:
  Reviewers: tyler
  Subscribers: ben
diff: {
  "phabricator.uri" : "https://phabricator.bitgo.com",
  - "history.immutable": true,
  - "arc.land.onto.default": "master",
  - "git.default-relative-commit": "origin/master"
  + "arc.land.onto.default": "master"
  }
  