diff -r 510c893a726f -r 11b5a5d2ca8b mercurial/commands.py
--- a/mercurial/commands.py	Tue Jul 05 14:28:55 2011 +0300
+++ b/mercurial/commands.py	Tue Jul 05 14:36:33 2011 +0300
@@ -3587,7 +3587,7 @@
 
     try:
         # ui.forcemerge is an internal variable, do not document
-        ui.setconfig('ui', 'forcemerge', opts.get('tool', ''))
+        repo.ui.setconfig('ui', 'forcemerge', opts.get('tool', ''))
         return hg.merge(repo, node, force=opts.get('force'))
     finally:
         ui.setconfig('ui', 'forcemerge', '')