Could we clean up the remote base, head, and orig branches after landing?

[takahirom]

I'm not sure if I'm using this tool correctly, but there are many leftover branches to remove after using ghstack.

https://github.com/takahirom/arbigent/branches/active

[ezyang]

Alas, it's how the tool works. You should use your favorite branch reaper to get rid of these branches when the PRs are closed.