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