--- a/mercurial/filemerge.py	Tue Oct 06 22:57:21 2015 -0700
+++ b/mercurial/filemerge.py	Wed Oct 07 00:01:16 2015 -0700
@@ -465,6 +465,9 @@
         binary = fcd.isbinary() or fco.isbinary() or fca.isbinary()
         symlink = 'l' in fcd.flags() + fco.flags()
         tool, toolpath = _picktool(repo, ui, fd, binary, symlink)
+        if tool in internals and tool.startswith('internal:'):
+            # normalize to new-style names (':merge' etc)
+            tool = tool[len('internal'):]
         ui.debug("picked tool '%s' for %s (binary %s symlink %s)\n" %
                    (tool, fd, binary, symlink))