Splitting a commit with rebase -i and keeping a commit message

I asked this on IRC and played with some of their ideas, but struck out with anything satisfying. I walked through [1] with the following setup:

  git init foo
  cd foo
  touch a.txt b.txt
  git add a.txt b.txt
  git commit -m "Initial checkin"
  echo "Modify A" >> a.txt
  git commit -am "Modified A"
  echo "Modify B" >> b.txt
  git commit -am "Modified B"
  echo "Modify A2" >> a.txt
  echo "Modify B2" >> b.txt
  git commit -am "Modified B"
  git commit -am "Long-bodied commit comment about b.txt changes"
  # whoops, just wanted B
  git rebase -i HEAD^^
  # change the "Added b.txt..." commit to "edit"
  git reset HEAD^  # pull the changes out of the pending commit
  git add a.txt
  git commit -m "Tweaked a.txt"
  git add b.txt
  git commit ${MAGIC_HERE}
  git rebase --continue

I haven't been able to figure out a good way to keep the "long-bodied commit comment" for the final commit where the ${MAGIC_HERE} is. Is there a right/easy way to go about pulling in the commit-message from the commit the rebase is transplanting?

-tkc

[1] http://git-scm.com/book/en/Git-Tools-Rewriting-History#Splitting-a-Commit