On branch swig-py3: Allow str return to svn_client_get_commit_log3_t callback
* subversion/bindings/swig/python/libsvn_swig_py/swigutil_py.c: (svn_swig_py_get_commit_log_func): Allow result as unicode object
* subversion/bindings/swig/python/tests/client.py: (SubversionClientTestCase.make_log_message_func): New helper function (SubversionClientTestCase.test_get_commit_log3_callback_accept_unicode): New test case (SubversionClientTestCase.test_get_commit_log3_callback_unicode_error): New test case