Detecting redundant commits

I am attempting to teach cherry-pick to handle redundant commits gracefully (via a new --skip-redundant-commits option) instead of aborting. However, I'm struggling a bit with how to check if the changes in a commit will become redundant when appied to the new HEAD.

I found diff_tree_sha1 which seems promising. Am I on the right track? If not, what's the best way to determine whether a commit object is redundant with respect to HEAD?

                           -David