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
Differential Revision: https://phab.mercurial-scm.org/D6458