windows: add context manager support to mixedfilemodewrapper

I stumbled into this in the next patch.  The difference between getting a
context manager capable object or not from vfs classes was as subtle as adding a
'+' to the file mode.

mercurial/windows.py

@@ -64,6 +64,12 b' class mixedfilemodewrapper(object):'
         object.__setattr__(self, r'_fp', fp)
         object.__setattr__(self, r'_lastop', 0)
 
+    def __enter__(self):
+        return self._fp.__enter__()
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        self._fp.__exit__(exc_type, exc_val, exc_tb)
+
     def __getattr__(self, name):
         return getattr(self._fp, name)