Comment 13 for bug 34540

I overdesigned it.

If we assume that branch ids never wrap around, we can make it much simpler:

 * Just delete the Branch database record in simple cases (when there is other Launchpad object attached to the branch).
 * Clean up filesystem data that is not associated to any branch in the database.
 * When the Branch is associated to other Launchpad objects, "merge" it in the way we can currently merge Persons. See https://blueprints.launchpad.net/launchpad-bazaar/+spec/branch-merge for rationale.

I do not have strong opinions on the process surrounding branch deletion. My gut feeling is that we should not overload last_modified_date as suggested by Tim, but instead of a dedicated timestamp, maybe date_pending_delete. But I will no push this point, as it is only a gut feeling.