[MERGE] Add ForeignBranch

[Aaron Bentley]

Jelmer Vernooij wrote:
> The attached patch adds a simple ForeignBranch class, as it is used at
> the moment as base class for the Branch classes in bzr-svn and bzr-git.

Hi, I don't want to be a blocker on this, but could you explain it a bit
more?

What's the "mapping" ivar?

Why are you defining dpush here?  Are you going to test whether branches
are ForeignBranches and then call dpush if they are?

Aaron