Fixed error for default repository defined in a `DbRepositoryProvider` rather than trac.ini. Refs #11436. Patch by Jun Omae.
 * Retrieve the default repository using `Environment.get_repository()` rather than trac.ini
 * Retrieve url of the new hack using `Repository.get_path_url()` and removed `[trachacks] subversion_base_url` option


git-svn-id: http://trac-hacks.org/svn/trachacksplugin/0.11@13488 7322e99d-02ea-0310-aa39-e9a107903beb