dirstate: force _checkexec to return a bool

posix.checkexec can return True, False, or None. The rust status
implementation expects a boolean, so make sure _checkexec returns a boolean.

Differential Revision: https://phab.mercurial-scm.org/D8432

mercurial/dirstate.py

@@ -180,7 +180,7 b' class dirstate(object):'
 
     @propertycache
     def _checkexec(self):
-        return util.checkexec(self._root)
+        return bool(util.checkexec(self._root))
 
     @propertycache
     def _checkcase(self):