--- a/mercurial/ancestor.py	Mon Sep 10 21:46:19 2018 +0900
+++ b/mercurial/ancestor.py	Mon Sep 10 21:54:40 2018 +0900
@@ -274,20 +274,26 @@
         visit = []
         heapq.heapify(visit)
         for r in initrevs:
-            for parent in parentrevs(r):
-                if parent not in seen:
-                    schedule(visit, -parent)
-                    see(parent)
+            p1, p2 = parentrevs(r)
+            if p1 not in seen:
+                schedule(visit, -p1)
+                see(p1)
+            if p2 not in seen:
+                schedule(visit, -p2)
+                see(p2)
 
     while visit:
         current = -nextitem(visit)
         if current < stoprev:
             break
         yield current
-        for parent in parentrevs(current):
-            if parent not in seen:
-                schedule(visit, -parent)
-                see(parent)
+        p1, p2 = parentrevs(current)
+        if p1 not in seen:
+            schedule(visit, -p1)
+            see(p1)
+        if p2 not in seen:
+            schedule(visit, -p2)
+            see(p2)
 
 class lazyancestors(object):
     def __init__(self, pfunc, revs, stoprev=0, inclusive=False):

--- a/tests/test-ancestor.py	Mon Sep 10 21:46:19 2018 +0900
+++ b/tests/test-ancestor.py	Mon Sep 10 21:54:40 2018 +0900
@@ -178,9 +178,9 @@
 # |
 # o  0
 
-graph = {0: [-1], 1: [0], 2: [1], 3: [1], 4: [2], 5: [4], 6: [4],
-         7: [4], 8: [-1], 9: [6, 7], 10: [5], 11: [3, 7], 12: [9],
-         13: [8]}
+graph = {0: [-1, -1], 1: [0, -1], 2: [1, -1], 3: [1, -1], 4: [2, -1],
+         5: [4, -1], 6: [4, -1], 7: [4, -1], 8: [-1, -1], 9: [6, 7],
+         10: [5, -1], 11: [3, 7], 12: [9, -1], 13: [8, -1]}
 
 def genlazyancestors(revs, stoprev=0, inclusive=False):
     print(("%% lazy ancestor set for %s, stoprev = %s, inclusive = %s" %