revset: made addset a private class

This class is not supposed to be used outside revset.py since it only
wraps content that is used by baseset typed classes.

It only gets created by revset operations or private methods.

mercurial/revset.py

@@ -2236,7 +2236,7 b' class lazyset(object):'
         return lazyset(self, lambda r: r not in x)
 
     def __add__(self, x):
-        return lazyset(addset(self, x))
+        return lazyset(_addset(self, x))
 
     def __nonzero__(self):
         for r in self:
@@ -2307,9 +2307,14 b' class orderedlazyset(lazyset):'
         self._subset.reverse()
         self._ascending = not self._ascending
 
-class addset(object):
-    """Wrapper structure for lazily adding two structures without losing much
+class _addset(object):
+    """Represent the addition of two sets
+
+    Wrapper structure for lazily adding two structures without losing much
     performance on the __contains__ method
+
+    This class does not duck-type baseset and it's only supposed to be used
+    internally
     """
     def __init__(self, revs1, revs2):
         self._r1 = revs1
@@ -2495,7 +2500,7 b' class spanset(object):'
             return orderedlazyset(self, lambda r: r not in x, ascending=False)
 
     def __add__(self, x):
-        return lazyset(addset(self, x))
+        return lazyset(_addset(self, x))
 
     def __len__(self):
         if not self._hiddenrevs: