ancestor: return early from _lazyancestorsiter() when reached to stoprev

There's no need to empty the heap.

mercurial/ancestor.py

@@ -281,7 +281,8 b' def _lazyancestorsiter(parentrevs, initr'
 
     while visit:
         current = -nextitem(visit)
-        if current >= stoprev:
+        if current < stoprev:
+            break
             yield current
             for parent in parentrevs(current):
                 if parent not in seen: