merge-package should have support for manipulating quilt v3 patch stacks

[John A Meinel]

Colin Watson described the proper steps for merging v3 packages.

1) Find the common packaging ancestor of the debian and ubuntu branches (taking care that sometimes bzr's common-ancestor algorithm would find a upstream revision as one of the common ancestors.)
2) Determine the stack of patches that are applied in that ancestor
3) pop ubuntu patches off, until we reach the stack in (2) [considering we may have removed some of the base patches]
4) merge the debian packaging branch, which will bring in some patches of its own
care needs to be taken as the series file will still mention the ubuntu patches, and if both debian and ubuntu introduce 1 new patch relative to base, you would get a conflict, which is a bit spurious
The content merge for the series file should ensure that applied patches are at the "bottom" of the stack, and that unapplied patches are at the "top".
5) then push back the ubuntu patches
pushing patches can conflict, can apply already applied changes, etc.