[Wilds] Make "Base Commit" parser test cases pass

Summary: Ref T13098. Chip away at test cases: update the tests for base rule parsing for the new WorkingCopy objects.

Test Plan: Ran arc unit, saw fewer test failures.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13098

Differential Revision: https://secure.phabricator.com/D19712