Rename "getWorkingCopy()" to "getWorkingCopyIdentity()" in Arcanist

Summary:
Depends on D21074. Ref T13490. Ref T11968. Before toolsets, Arcanist has a "WorkingCopyIdentity" object. After toolsets, it has a "WorkingCopy" object.

Most workflows don't access either one, so make a slightly-breaking API change to simplify the transition.

• "getWorkingCopy()" now returns a modern object (a "WorkingCopy").
• "getWorkingCopyIdentity()" now returns an older object (a "WorkingCopyIdentity").

This isn't backward-compatible, but out-of-date code should get an explicit failure with clear resolution steps.

Test Plan: Ran "arc lint", "arc branch", and a few other commands. Grepped for "getWorkingCopy()".

Maniphest Tasks: T13490, T11968

Differential Revision: https://secure.phabricator.com/D21075