tag.target_type returns a symbol, not string.
s/rugged/repo in TreeBuilder section.
No method get_entry for index.
Fix Rugged::Reference part.

1. ref.type returns a symbol instead of string.
2. ref.set_target instead of ref.target=
3. Reference entry has keys id_old and id_new instead of oid_*
repo instead of @repo in Branch section.