Support stacked PRs

Something like 2nd approach: https://thoughttrain.app/l/a_better_model_for_stacked_github_pull_requests_ti_34433 but allow creation of multiple branches for a given issue, supporting their ordering

Flow:

1. Create first branch B1
2. When B1 is complex enough, let ghh branch out B2
3. Support "B1 merged, update B2 via rebase"
4. Support "B1 updated with new commits, recreate B2"