Add support for `gt downstack get` command

[danerwilliams]

https://graphite.dev/docs/collaborating-on-a-stack
https://graphite.dev/docs/downstack

This command was deleted in this open source version of graphite due to it being dependent on Graphite's API.

We need to add support for this that ideally just uses git commands, but if necessary could use the GitHub API as well.

All we really need to do is figure out how to get a list of the downstack branches in liu of the method Graphite's original implementation used to do this, which relied on fetching from their API:
https://github.com/danerwilliams/graphite-cli/blob/9d4e1f5c496e0d448714adb6f90ba14a8868b418/apps/cli/src/actions/sync/get.ts#L32-L44