rust-dirstatemap: add missing @propertycache

While investigating a regression on `hg update` performance introduced by the
Rust `dirstatemap`, two missing `@propertycache` were identified when comparing
against the Python implementation. This adds back the first one, that has
no observable impact on behavior. The second one (`nonnormalset`) is going to
be more involved, as the caching has to be done from the Rust side of things.

Differential Revision: https://phab.mercurial-scm.org/D8047

mercurial/dirstate.py

@@ -1849,7 +1849,7 b' if rustmod is not None:'
             nonnorm, otherparents = self._rustmap.nonnormalentries()
             return nonnorm
 
-        @property
+        @propertycache
         def otherparentset(self):
             nonnorm, otherparents = self._rustmap.nonnormalentries()
             return otherparents