[MERGE] Add base classes for foreign branches.

[Jelmer Vernooij]

The attached merge request adds base classes for foreign branch
plugins (such as bzr-svn and bzr-git). It is extracted from the
"bzr-foreign" branch that is included in both bzr-svn and bzr-git. 

Other than providing some base classes that all foreign branch
implementations need, this patch also adds a custom revision property
handler for "log" that will display the foreign branches revision
identifier - e.g. {"svn revno": "42 (on /trunk)"} for svn or {"git
commit": "a3b86a4823b68ebae87b623"} for git.

Cheers,

Jelmer
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