py3: make sure we return strings from __str__ and __repr__

On Python 3:

>>> class abc:
...     def __repr__(self):
...             return b'abc'
...
>>> abc()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __repr__ returned non-string (type bytes)

>>> class abc:
...     def __str__(self):
...             return b'abc'
...
>>> str(abc())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __str__ returned non-string (type bytes)

So the __str__ and __repr__ must return strings.

mercurial/context.py

@@ -77,7 +77,7 b' class basectx(object):'
         return self.rev()
 
     def __repr__(self):
-        return "<%s %s>" % (type(self).__name__, str(self))
+        return r"<%s %s>" % (type(self).__name__, str(self))
 
     def __eq__(self, other):
         try:
@@ -1403,7 +1403,7 b' class committablectx(basectx):'
             self._extra['branch'] = 'default'
 
     def __str__(self):
-        return str(self._parents[0]) + "+"
+        return str(self._parents[0]) + r"+"
 
     def __nonzero__(self):
         return True