[MERGE] Add a ForeignRepository base class

[Jelmer Vernooij]

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.

Cheers,

Jelmer

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