Check for untracked files in order to fire untracked file prompt

Summary: Fixes T7465. I think I just missed this when untangling the old logic.

Test Plan: Ran arc diff with only untrakced files, saw warning.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7465

Differential Revision: https://secure.phabricator.com/D11990