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

hgext/convert/subversion.py

@@ -1333,7 +1333,7 b' class svn_sink(converter_sink, commandli'
                 rev = self.commit_re.search(output).group(1)
             except AttributeError:
                 if not files:
-                    return parents[0] if parents else None
+                    return parents[0] if parents else 'None'
                 self.ui.warn(_('unexpected svn output:\n'))
                 self.ui.warn(output)
                 raise error.Abort(_('unable to cope with svn output'))