revset: use 'next()' to detect end of iteration in 'limit'

The 'next()' built-in can return a default value, allow to get rid of the
confusing try/except code flow.

mercurial/revset.py

@@ -1145,12 +1145,11 b' def limit(repo, subset, x):'
     result = []
     it = iter(os)
     for x in xrange(lim):
-        try:
-            y = it.next()
-            if y in ss:
-                result.append(y)
-        except (StopIteration):
+        y = next(it, None)
+        if y is None:
             break
+        elif y in ss:
+            result.append(y)
     return baseset(result)
 
 def last(repo, subset, x):