Rename "getWorkingCopy()" to "getWorkingCopyIdentity()" in Arcanist
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This isn't backward-compatible, but out-of-date code should get an explicit failure with clear resolution steps.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()".
Differential Revision: https://secure.phabricator.com/D21075