Trivially update "arc branch/feature" and "arc browse" for Toolsets

Summary:
Ref T11968. Ref T13490. These are the workflows which currently use the intermediate-level hardpoint code (which made hardpoints formal, but didn't do the yield stuff).

Move toward updating them by doing some basic bookkeeping, with a few compatibility adjustments to the parent Workflow class.

Test Plan: Ran "arc branch" and "arc browse" with various arguments.

Maniphest Tasks: T13490, T11968

Differential Revision: https://secure.phabricator.com/D21074