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