windows: ensure mixedfilemodewrapper fd doesn't escape by entering context mgr

Otherwise it seems that the special read and write handling would be bypassed.

mercurial/windows.py

@@ -70,7 +70,8 b' class mixedfilemodewrapper(object):'
         object.__setattr__(self, r'_lastop', 0)
 
     def __enter__(self):
-        return self._fp.__enter__()
+        self._fp.__enter__()
+        return self
 
     def __exit__(self, exc_type, exc_val, exc_tb):
         self._fp.__exit__(exc_type, exc_val, exc_tb)