revset: added __add__ method to _addset

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

mercurial/revset.py

@@ -2425,6 +2425,17 b' class _addset(object):'
             return orderedlazyset(self, filterfunc, ascending=self._ascending)
         return lazyset(self, filterfunc)
 
+    def __add__(self, other):
+        """When both collections are ascending or descending, preserve the order
+        """
+        kwargs = {}
+        if self._ascending is not None:
+            if self.isascending() and other.isascending():
+                kwargs['ascending'] = True
+            if self.isdescending() and other.isdescending():
+                kwargs['ascending'] = False
+        return _addset(self, other, **kwargs)
+
     def _iterator(self):
         """Iterate over both collections without repeating elements