svnwrap: remove dead code Some member functions of SubversionRepo were unused, and removing them frees other wrappers from adding possibly incorrect implementations of them. Two methods, `tags_at_rev' and `_get_copy_source' were completely unused and could easily be removed. Another two methods, `branches' and `tags' had explicit tests for them but weren't used in the code proper; they were removed too. The START property was unnecessary and could be removed with a tiny refactoring. No functionality change.

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