py3: implement __bytes__ for committablectx

Before this method, calling bytes on workingctx or memctx calls
basectx.__bytes__ since the magic method was not defined for this class. When it
calls the method from basectx class, it returns TypeError because None is passed
into it.

After this commit `hg update -C` works on Python 3 if eol is not enabled.

mercurial/context.py

@@ -1405,6 +1405,9 b' class committablectx(basectx):'
     def __str__(self):
         return str(self._parents[0]) + r"+"
 
+    def __bytes__(self):
+        return bytes(self._parents[0]) + "+"
+
     def __nonzero__(self):
         return True