merge & tree shape alteration hook for packaging

For working with packages that are maintained with separate patches in debian, it would be *awesome* to be able to have regular branches and collapse the branches into a set of patches just-in-time.

There are two main things missing to do this:
 - a list of branches that should become patches (both pipelines and looms can do this though)
 - A hook during the switch process so that switching to the main packaging branch can cause the patches to be created/updated.

This bug is about that hook.

I don't know quite what it needs to look like, but it needs to be fairly flexible as there are multiple different sorts of output (quilt, dpatch, heck, even rpm spec files) that would be needed.

What it needs to permit is a little complex:
 - changes to files (and tree shape) that are not coming from an upstream update need to get converted to parts of patches
 - changes to files and tree shape coming from upstream should end up in the upstream files.

As an example, consider the following packaging loom/pipe:
ubuntu
patch2
patch1
upstream

When an upstream release happens all four branches may change: upstream (duh). patch1 to deal with a conflict in upstream, and patch2 likewise.
Afte the merge to ubuntu, all the files present in upstream should be unmodified vis-a-vis upstream, and there should be two updated patches in the debian/ subtree.

It may be possible to do this via command decoration, but doing it as a hook would let it work for GUI's etc too.