Fix arc which output

Summary: Fixes T8231. This was derped in some recent refactoring.

Test Plan: Ran arc which and saw expected output.

Reviewers: avivey, epriestley, Blessed Reviewers

Reviewed By: avivey, epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8231

Differential Revision: https://secure.phabricator.com/D12890