py3: ensure the proxied Windows fd doesn't escape by entering context manager

The purpose of the proxy class is to provide the `name` attribute which contains
the file path.  But in tests that used a context manager, it still blew up
complaining that 'int' doesn't have a 'startswith' function.

mercurial/windows.py

@@ -132,7 +132,10 b' class fdproxy(object):'
         self._fp = fp
 
     def __enter__(self):
-        return self._fp.__enter__()
+        self._fp.__enter__()
+        # Return this wrapper for the context manager so that the name is
+        # still available.
+        return self
 
     def __exit__(self, exc_type, exc_value, traceback):
         self._fp.__exit__(exc_type, exc_value, traceback)