--- a/mercurial/copies.py	Sat May 06 04:51:25 2017 +0530
+++ b/mercurial/copies.py	Fri Feb 10 16:56:29 2017 -0800
@@ -637,7 +637,7 @@
         return
 
     of = None
-    seen = set([f])
+    seen = {f}
     for oc in getfctx(f, m1[f]).ancestors():
         ocr = oc.linkrev()
         of = oc.path()