unmerging feature branches

Dear list,

Let's say I developed a feature Foo on a branch off master, and at some point I merged it back into master (commit M) and published the repo. Since M, a number of commits have been made onto master.

Now I woul like to undo the merge.

I could rebase (M+1)..master onto M^ (on the former master branch), but that would orphan the commits between the merge point and the tip of master, which others are tracking.

I'd love to have git-revert, but that cannot undo a multi-parent commit.

I could git-revert every commit on the feature branch between the branch point and the merge point, even squash them into a single commit, but that is a lot of work.

Are there any other methods? Is it conceivable to let git-revert revert a merging commit if you tell it somehow which of the two (or more) parents are the ones you want undone, meaning that you'd like to keep the others?