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