Fixes T13459. Currently, the "experimental" branch can not "arc upgrade" itself, but this is a reasonable thing to do; allow it to.
- Maniphest Tasks
- T13459: Allow "arc upgrade" to upgrade the "experimental" branch
- rARC039144ea24ba: Allow "arc upgrade" to work on the "experimental" branch
Ran "arc upgrade" on "experimental". (This has some bootstrapping issues; will verify after landing.)