[MERGE] Support dpushing an empty branch.

[Aaron Bentley]

Aaron Bentley has voted comment.
Status is now: Semi-approved
Comment:
I don't understand what makes it difficult to test this.  Also, 
repo.revision_tree *will* return an empty tree, and why does this need 
to operate on inventories rather than trees?

For details, see: 
http://bundlebuggy.aaronbentley.com/project/bzr/request/%3C49DFBE20.2080209%40vernstok.nl%3E
Project: Bazaar