Pull git upstream-path logic into a separate class

Summary:
Ref T9661. I need to reuse this to fix the complex workflow described in T9661 where we need to follow multiple paths to the upstream and cascade updates across them.

Pull the logic into a separate class to make this easier and less copy/pastey.

This shouldn't change any behavior.

Test Plan: Ran arc land --preview from detached head, remote-tracking branch, non-tracking branch, local-tracking branch. Selection of target/remote seemed correct in all cases.

Reviewers: chad

Reviewed By: chad

Subscribers: edibiase

Maniphest Tasks: T9661

Differential Revision: https://secure.phabricator.com/D14360