[MERGE] Add a ForeignRepository base class

[Jelmer Vernooij]

On Fri, 2008-12-05 at 04:26 +0100, Jelmer Vernooij wrote:
> This patch adds a very simple ForeignRepository class that just adds
> some stubs and convenience implementations for a few bzr-specific
> things for now. 
> 
> This code is currently already used in both bzr-svn and bzr-git, but it 
> would be nice to have in bzrlib to avoid code duplication and to allow 
> improved errors for users of foreign branches. 
> 
> For example, it would be nice to be able to tell users when they are
> pulling from a branch with a newer mapping than was used for
> the revisions they have locally instead of giving them a generic
> DivergedBranches error like we do now.
Oh, and it should help in providing a testsuite that can be used by
multiple Foreign* implementations.

Cheers,

Jelmer