support remotely shipped nested branches

[kevgo]

I have nested feature branches:

main
 \
  feature1
   \
    feature2

Git Town knows the branch hierarchy.

I submit a PR for feature1, the maintainer merges my MR and deletes the branch on the remote.

On the next sync, Git Town should update feature2 to have the parent main.

[wederbrand]

I came here to say that this is exactly my workflow and a much wanted feature.

[ruudk]

This would also allow the PR author to merge its own PR on the GitHub interface, without having to use ship.

[kevgo]

This should work in the upcoming v10 release: https://github.com/git-town/git-town/blob/main/features/sync/current_branch/feature_branch/tracking_branch_deleted/features/multiple_ancestors_shipped.feature