[MERGE] Add BranchBzrDirInter.push()

[Robert Collins]

Robert Collins has voted tweak.
Status is now: Conditionally approved
Comment:
It would be nice to have tests for this; it is mainly moved code so 
perhaps finding and moving some tests (to per_bzrdir) would be doable. 
If not then at least a couple of smoke tests please.

Perhaps 'push_branch' rather than just 'push'.

For details, see: 
http://bundlebuggy.aaronbentley.com/project/bzr/request/%3C20090401141753.GA16842%40vernstok.nl%3E
Project: Bazaar