Separate "browse ref -> commit" and "commit -> upstream" hardpoints from arc browse workflow

Summary:
Ref T10895. Currently, the "browse as a commit" code does these lookups, but the code can't be reused.

For T10895, I want to introduce "browse as revision", but it will need to do the same ref lookup. Separate this as a hardpoint so the code can be shared via hardpoint/ref infrastructure.

Test Plan: Ran arc browse master and similar commands.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10895

Differential Revision: https://secure.phabricator.com/D16929