--- a/mercurial/cmdutil.py	Thu Jul 19 23:22:05 2018 -0700
+++ b/mercurial/cmdutil.py	Fri Jul 27 14:49:45 2018 -0700
@@ -2428,7 +2428,7 @@
         if len(old.parents()) > 1:
             # ctx.files() isn't reliable for merges, so fall back to the
             # slower repo.status() method
-            files = set([fn for st in repo.status(base, old)[:3]
+            files = set([fn for st in base.status(old)[:3]
                          for fn in st])
         else:
             files = set(old.files())