_addset: add a __len__ method

Back in the time where repo.revs(...) returned a list, calling `len(...)` on the
result was quite common. We reinstall this on _addset.

There is absolutely no easy way to test this from the command line. The commands
using this in the evolve extension will eventually land into core.

mercurial/revset.py

@@ -2464,6 +2464,9 b' class _addset(_orderedsetmixin):'
         self._ascending = ascending
         self._genlist = None
 
+    def __len__(self):
+        return len(self._list)
+
     @util.propertycache
     def _list(self):
         if not self._genlist: