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