Allow "arc upgrade" to work on the "experimental" branch

Summary: Fixes T13459. Currently, the "experimental" branch can not "arc upgrade" itself, but this is a reasonable thing to do; allow it to.

Test Plan: Ran "arc upgrade" on "experimental". (This has some bootstrapping issues; will verify after landing.)

Maniphest Tasks: T13459

Differential Revision: https://secure.phabricator.com/D20916