always set new to true for fresh instances

Summary:
Also, always return `new` even when it's false.

Depends on D913912.

Test Plan: `arc unit --everything`

Reviewers: wez

Reviewed By: wez

Differential Revision: https://phabricator.fb.com/D915984

--HG--
extra : rebase_source : 982b2f1ca8ebecba13769003e8ac945ccddb72e1
extra : amend_source : f984e7a659a92ec067505e2162bb8a73b6dd0ff1