How to selectively recreate merge state?

Let's say you initiate a merge:

$ git merge topic

And this merge results in conflicts in two files, foo and bar. You
resolve the conflicts in both files, but then decide you don't like
how you resolved bar.

How do you set the index and working-copy back to the state it was
immediately after doing the merge for bar, while leaving the merge
resolution alone for foo?

j.