Bring a "pro" WorkingCopyState ref to "master"

Summary:
Ref T11968. Continue bringing modern yield-based hardpoint code into "master" in the parallel "Pro" classtree.

Adds "working-copy(commit-hash)" as an inspectable ref.

Test Plan: Inspected working copy refs, saw them resolve revisions by commit hash and commit message.

Maniphest Tasks: T11968

Differential Revision: https://secure.phabricator.com/D21079