generatorset: use 'next()' to simplify the code

generatorset: use 'next()' to simplify the code

The 'next()' built-in accept a default value. This remove the needs to check if
self non-empty before returning a value.

--- a/mercurial/revset.py	Sun May 17 18:00:38 2015 -0700
+++ b/mercurial/revset.py	Sun May 17 18:06:09 2015 -0700
@@ -3322,9 +3322,7 @@
             for x in self._consumegen():
                 pass
             return self.first()
-        if self:
-            return it().next()
-        return None
+        return next(it(), None)
 
     def last(self):
         if self._ascending:
@@ -3336,9 +3334,7 @@
             for x in self._consumegen():
                 pass
             return self.first()
-        if self:
-            return it().next()
-        return None
+        return next(it(), None)
 
     def __repr__(self):
         d = {False: '-', True: '+'}[self._ascending]