--- a/mercurial/dagop.py	Sat Jun 17 22:33:23 2017 +0900
+++ b/mercurial/dagop.py	Sun Jun 18 17:16:02 2017 +0900
@@ -26,6 +26,9 @@
     else:
         cut = None
     cl = repo.changelog
+
+    # load input revs lazily to heap so earlier revisions can be yielded
+    # without fully computing the input revs
     revs.sort(reverse=True)
     irevs = iter(revs)
     h = []