addset: fix `first` and `last` on sorted addset (issue4426)

The lazy sorting were not enforced on addset. This was made visible through MQ.

mercurial/revset.py

@@ -2642,14 +2642,15 b' class addset(abstractsmartset):'
             self._ascending = not self._ascending
 
     def first(self):
-        if self:
-            return self._list.first()
+        for x in self:
+            return x
         return None
 
     def last(self):
-        if self:
-            return self._list.last()
-        return None
+        self.reverse()
+        val = self.first()
+        self.reverse()
+        return val
 
 class generatorset(abstractsmartset):
     """Wrap a generator for lazy iteration

tests/test-mq.t

@@ -1581,3 +1581,19 b' Test that secret mq patch does not break'
   tip	[0-9a-f]{40} (re)
 
   $ cd ..
+
+Test interraction with revset (issue4426)
+
+  $ hg init issue4426
+  $ cd issue4426
+
+  $ echo a > a
+  $ hg ci -Am a
+  adding a
+  $ echo a >> a
+  $ hg ci -m a
+  $ echo a >> a
+  $ hg ci -m a
+  $ hg qimport -r 0::
+
+  $ cd ..