Allow upgrading in branch stable

Summary: close T9043. This still allows for one dir to be in master and the other in stable, but hopefully that's not going to be a problem.

Test Plan: Clone arc/libph, checkout stable, run arc upgrade.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: cburroughs, epriestley

Maniphest Tasks: T9043

Differential Revision: https://secure.phabricator.com/D14076