[MERGE] Support dpushing an empty branch.

[Michael Hudson]

Aaron Bentley wrote:
> 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?

revision_trees() won't though, which is something I keep meaning to fix...

Cheers,
mwh