diff -r d2554ef04d26 -r 5caae380ff90 mercurial/filemerge.py
--- a/mercurial/filemerge.py	Tue Oct 31 06:39:38 2017 +0530
+++ b/mercurial/filemerge.py	Thu Oct 26 11:07:06 2017 -0700
@@ -116,7 +116,7 @@
             continue
         p = util.lookupreg(k, _toolstr(ui, tool, "regname"))
         if p:
-            p = util.findexe(p + _toolstr(ui, tool, "regappend"))
+            p = util.findexe(p + _toolstr(ui, tool, "regappend", ""))
             if p:
                 return p
     exe = _toolstr(ui, tool, "executable", tool)