revset: fix first and last for generatorset (issue4465)

The code was just plain wrong.

mercurial/revset.py

@@ -2804,7 +2804,7 b' class generatorset(abstractsmartset):'
                 pass
             return self.first()
         if self:
-            return it.next()
+            return it().next()
         return None
 
     def last(self):
@@ -2818,7 +2818,7 b' class generatorset(abstractsmartset):'
                 pass
             return self.first()
         if self:
-            return it.next()
+            return it().next()
         return None
 
 def spanset(repo, start=None, end=None):

tests/test-issue619.t

@@ -28,3 +28,9 b' Bogus fast-forward should fail:'
   abort: merging with a working directory ancestor has no effect
   [255]
 
+Even with strange revset (issue4465)
+
+  $ hg merge ::.
+  abort: merging with a working directory ancestor has no effect
+  [255]
+