dirstate: revert change to Rust binding

The Rust binding uses its own class and is not derived from the regular
implementation. As such, it didn't get _nodelen.

Differential Revision: https://phab.mercurial-scm.org/D8849

mercurial/dirstate.py

@@ -1835,7 +1835,7 b' if rustmod is not None:'
             if not self._parents:
                 try:
                     fp = self._opendirstatefile()
-                    st = fp.read(2 * self._nodelen)
+                    st = fp.read(40)
                     fp.close()
                 except IOError as err:
                     if err.errno != errno.ENOENT: