Fix WorkingCopy step to read correct commit variables

Summary: Ref T9252. This variable was always wrong but we fell back to just resetting to `HEAD` before. Use the correct variable name.

Test Plan: Verified variable name.

Reviewers: chad, hach-que

Reviewed By: hach-que

Maniphest Tasks: T9252

Differential Revision: https://secure.phabricator.com/D14224

src/applications/harbormaster/step/HarbormasterLeaseWorkingCopyBuildStepImplementation.php

···
 
     $repository = $repositories[$repository_phid];
 
-    $commit = idx($variables, 'repository.commit');
+    $commit = idx($variables, 'buildable.commit');
     $ref_uri = idx($variables, 'repository.staging.uri');
     $ref_ref = idx($variables, 'repository.staging.ref');
     if ($commit) {