diff -r e1c586b9a43c -r 5744ceeb9067 mercurial/filemerge.py
--- a/mercurial/filemerge.py	Mon Nov 21 15:04:42 2022 -0500
+++ b/mercurial/filemerge.py	Sun Nov 20 23:09:12 2022 -0500
@@ -158,7 +158,7 @@
             continue
         p = util.lookupreg(k, _toolstr(ui, tool, b"regname"))
         if p:
-            p = procutil.findexe(p + _toolstr(ui, tool, b"regappend", b""))
+            p = procutil.findexe(p + _toolstr(ui, tool, b"regappend"))
             if p:
                 return p
     exe = _toolstr(ui, tool, b"executable", tool)