py3: fix test-convert-svn-sink.t In cases where the root commit is empty commit, None will be returned as parents. This was implemented by 2c13e91ede6e. This breaks test on py3 because `b'%s' % None` does not work. It does not matter whether we return `None` or `'None'` as we skipped converting to svn step by doing an early return. So let's return `'None'`. I tried to patch all the users to convert `None` to `'None'`, but there were more users than I expected. I hit 3 of them and decided to fix it this way around. Differential Revision: https://phab.mercurial-scm.org/D6458