Allow arc unit --everything to work without a base commit

Summary: Fixes T2461. Similarly to arc lint --everything, arc unit --everything should work without a base commit.

Test Plan: Removed the .git/arc/default-relative-commit file and ran arc unit --everything.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T2461

Differential Revision: https://secure.phabricator.com/D11459