Make "no working copy" a more explicit "arc" VCS

Summary:
Some commands (like get-config) do not require a working copy at all. Recent changes prevented these commands from running outside a VCS working copy.

Let null mean "no working copy".

See also https://github.com/phacility/phabricator/issues/800.

Test Plan:

• Ran arc get-config from outside a working copy.
• Ran arc list from outside a working copy, got an error.
• Ran arc commit in a Git working copy, got an error.

Reviewers: joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11643