diff -r 6aa7dcae6bd8 -r 47fd57f650e5 mercurial/revset.py
--- a/mercurial/revset.py	Thu Jan 30 16:03:18 2014 -0800
+++ b/mercurial/revset.py	Thu Jan 30 16:47:29 2014 -0800
@@ -992,7 +992,7 @@
     # i18n: "merge" is a keyword
     getargs(x, 0, 0, _("merge takes no arguments"))
     cl = repo.changelog
-    return baseset([r for r in subset if cl.parentrevs(r)[1] != -1])
+    return lazyset(subset, lambda r: cl.parentrevs(r)[1] != -1)
 
 def branchpoint(repo, subset, x):
     """``branchpoint()``