Rebasing a merged branch

Hi!

If I slightly modify the example from the git-rebase manual page to look like this:

   o---o---o---o  master
          \
           o---o---o---o---o  topicA
            \     /
             A---B---C---D  topicB

(topicA has merged "B" into its history; its first-parent from the line of "o"s).

If I now do a "git rebase --onto master topicA topicB", I only get commit C and D, as it sees A and B as being part of both branches.

Is there a way to make git rebase pick up A, B, C and D (and only them)?

I.e., I would like "all commits on topicB which are not in topicA's --first-parent history".

I eventually came up with

   git rebase --onto master $(git rev-list topicA ^topicB | tail -1) topicB

but am thinking that it ought to be expressable in a simpler way?