baseset: make `_set` a property cache

This will remove the need for `baseset.set()`.

mercurial/revset.py

@@ -2313,10 +2313,13 b' class baseset(abstractsmartset):'
         if not isinstance(data, list):
             data = list(data)
         self._list = data
-        self._set = None
         self._ascending = None
 
     @util.propertycache
+    def _set(self):
+        return set(self._list)
+
+    @util.propertycache
     def _asclist(self):
         asclist = self._list[:]
         asclist.sort()