[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