py3: make sure __repr__ returns a str

# skip-blame because just r'' prefix

Differential Revision: https://phab.mercurial-scm.org/D2652

mercurial/context.py

@@ -748,7 +748,7 b' class basefilectx(object):'
     __str__ = encoding.strmethod(__bytes__)
 
     def __repr__(self):
-        return "<%s %s>" % (type(self).__name__, str(self))
+        return r"<%s %s>" % (type(self).__name__, str(self))
 
     def __hash__(self):
         try: