--- a/mercurial/patch.py	Sat Jul 11 00:31:21 2020 +0530
+++ b/mercurial/patch.py	Thu Jul 09 18:48:55 2020 -0700
@@ -2666,7 +2666,11 @@
     prefetchmatch = scmutil.matchfiles(
         repo, list(modifiedset | addedset | removedset)
     )
-    scmutil.prefetchfiles(repo, [ctx1.rev(), ctx2.rev()], prefetchmatch)
+    revmatches = [
+        (ctx1.rev(), prefetchmatch),
+        (ctx2.rev(), prefetchmatch),
+    ]
+    scmutil.prefetchfiles(repo, revmatches)
 
     def difffn(opts, losedata):
         return trydiff(