Implement dpush via sexy APIs

[Jelmer Vernooij]

Hi Aaron,

Aaron Bentley wrote:
> Here's a rough, working implementation of the approach I was suggesting
> in IRC.
>
> I hope you see why I think using TreeTransform is even simpler than
> using Tree.apply_inventory_delta.
Thanks, I agree that is both simpler, shorter and better performing on
large trees with a small number of changes. I'll see if I can come up
with a better version of my patch based on yours in the next couple of days.

Cheers,

Jelmer