revset: added ascending and descending methods to _addset

This methods are intended to duck-type baseset, so we will still have _addset
as a private class but will be able return it without wrapping it into an
orderedlazyset or a lazyset.

mercurial/revset.py

@@ -2397,6 +2397,22 b' class _addset(object):'
             return orderedlazyset(self, condition, ascending=self._ascending)
         return lazyset(self, condition)
 
+    def ascending(self):
+        if self._ascending is None:
+            self.sort()
+            self._ascending = True
+        else:
+            if not self._ascending:
+                self.reverse()
+
+    def descending(self):
+        if self._ascending is None:
+            self.sort(reverse=True)
+            self._ascending = False
+        else:
+            if self._ascending:
+                self.reverse()
+
     def _iterator(self):
         """Iterate over both collections without repeating elements