transaction: tr._names are actually bytes, use byte string to join them

Looks like this is never checked in the tests? But trying e.g. `print(tr)` by
hand before this change would give:

  TypeError: sequence item 0: expected str instance, bytes found

Looks like tr._names are always bytes (although this isn't actually enforced,
but maybe at some point it will at least be type checked e.g. by pytype).

mercurial/transaction.py

@@ -314,7 +314,7 b' class transaction(util.transactional):'
         self._abortcallback = {}
 
     def __repr__(self):
-        name = '/'.join(self._names)
+        name = b'/'.join(self._names)
         return '<transaction name=%s, count=%d, usages=%d>' % (
             name,
             self._count,