windows: ensure pure posixfile fd doesn't escape by entering context manager

There are tests in test-revlog-mmapindex.t and test-rebase-mq-skip.t that are
fixed by this, but we usually don't use --pure on Windows.  For whatever reason,
the remaining --pure failures are various errors like  $ENOTDIR$ and "Access is
denied" have a trailing '.'.

mercurial/pure/osutil.py

@@ -267,7 +267,8 b' else:'
             return self._file.__setattr__(name, value)
 
         def __enter__(self):
-            return self._file.__enter__()
+            self._file.__enter__()
+            return self
 
         def __exit__(self, exc_type, exc_value, exc_tb):
             return self._file.__exit__(exc_type, exc_value, exc_tb)