mercurial/filemerge.py

--- a/mercurial/filemerge.py	Tue May 22 18:10:37 2018 +0200
+++ b/mercurial/filemerge.py	Thu May 24 15:56:47 2018 -0600
@@ -574,7 +574,7 @@
             try:
                 # avoid cycle cmdutil->merge->filemerge->extensions->cmdutil
                 from . import extensions
-                mod = extensions.loadpath(toolpath, 'hgmerge.%s' % scriptfn)
+                mod = extensions.loadpath(toolpath, 'hgmerge.%s' % tool)
             except Exception:
                 raise error.Abort(_("loading python merge script failed: %s") %
                                   toolpath)