Git feature request: --amend older commit

With git's "commit frequently" style, I often find that I end up with a commit that includes a typo in a comment or I forgot one call site when updating functions or something.

And it's a few commits later before I notice the simple oops.

This is of course fixable by making a commit, rebase -i HEAD~4 (or whatever), and marking the fixup for squashing into the previous commit.

But it would be really handy if there were a one-step command for doing this.

Something like "git commit --fixup HEAD~3", where "git commit --fixup HEAD" would be equivalent to "git commit --amend".

It would be fine if it were implemented using rebase -i and you had to use "git rebase --continue" to recover from a conflict. And history had to be linear from the fixup point to HEAD.

The only thing I'd wish for, that rebase -i doesn't support, is a commit with a dirty tree. (Because often the typo is noticed in the middle of further development.) But if I have to manually stash & pop, that would be good enough.

Talking with some friends, they all say "yeah, I would really use that feature". So I figured I'd mention it here.