diff -r b10a944c0486 -r a54a866349ed mercurial/filemerge.py
--- a/mercurial/filemerge.py	Tue Mar 15 13:40:45 2022 -0700
+++ b/mercurial/filemerge.py	Tue Mar 15 13:31:39 2022 -0700
@@ -755,8 +755,7 @@
         # Remove the .orig to make syntax-highlighting more likely.
         if localoutputpath.endswith(b'.orig'):
             localoutputpath, ext = os.path.splitext(localoutputpath)
-        localdata = util.readfile(localpath)
-        files.append((b"local", localoutputpath, localdata))
+        files.append((b"local", localoutputpath, backup.data()))
 
     with _maketempfiles(files) as temppaths:
         basepath, otherpath = temppaths[:2]