getset: check if an object is a baseset using `isascending` instead of `set`

The `set()` method is going away.

mercurial/revset.py

@@ -235,7 +235,7 b' def getset(repo, subset, x):'
     if not x:
         raise error.ParseError(_("missing argument"))
     s = methods[x[0]](repo, subset, *x[1:])
-    if util.safehasattr(s, 'set'):
+    if util.safehasattr(s, 'isascending'):
         return s
     return baseset(s)