Fix `arc diff --raw` when run outside a repository

Summary: Fixes T5071. If you run `arc diff --raw` from some arbitrary, non-repository directory we incorrectly try to read information out of the working copy. Even if this information is available, `arc diff --raw` should not assume it's accurate (there's no guarantee the diff comes from the working copy).

Test Plan: Ran `arc diff --raw` in an arbitrary directory.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5071

Differential Revision: https://secure.phabricator.com/D9142