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

src/workflow/ArcanistWorkflow.php

@@ -881,7 +881,7 @@ final public function requireCleanWorkingCopy() {
     }
 
     $should_commit = false;
-    if ($unstaged || $uncommitted) {
+    if ($untracked || $unstaged || $uncommitted) {
 
       // NOTE: We're running this because it builds a cache and can take a
       // perceptible amount of time to arrive at an answer, but we don't want