commit-builder-based InterRepository implementation

[Jelmer Vernooij]

bzr-svn implements push basically by obtaining a SvnCommitBuilder and replaying the changes in the revision that has to be pushed to that. I think this functionality should be moved to bzr core, because it is also useful for other implementations (such as bzr-git).

[Jelmer Vernooij (jelmer)]

I should probably add, a commit-builder-based push is probably not the fastest implementation in all cases, but it's "free" and better than nothing. more specific, faster, implementations can always be added, of course.