How to interactively rebase-and-reword nth commit?

[Konstantin Kharlamov <hi-angel@yandex.ru>]

I think, one of the most frequent git features used is `rebase -i`. There's a common
workflow I think everyone knows: you have commits 1, 2, 3, then you decide "Nah, 2nd
commit needs a change", so you do `git rebase -i HEAD~2`, then in popped up editor
you modify the `pick` on the first line to become `edit`, then you do the change,
then `git rebase --continue`.

The boilerplate part here: even though you know that you want to edit HEAD~2, there
is no way to tell git that right away. Every time you have to launch editor, edit the
line manually, then save it, close it.

I have seen here some discussions about improving that, someone even posted patches,
but I'm not aware if it went anywhere. So I created 2 years ago a shell wrapper
`rebase-at`¹, which upon called as `rebase-at e HEAD~2`, does the thing described
above automatically. Under the hood I simply substitute EDITOR with `sed` command
that replaces `pick` on the first line (the HEAD~2 commit) with `e`. If used with
shell autocompletion, it is now practically instantaneous.

I'm almost happy with `rebase-at`, except I don't know of any way to make it work
with `reword` git action. You see, "rewording a commit" requires to run EDITOR twice:
first to substitute `pick` with `reword`, and then to actually edit the commit
message. But since EDITOR was substituted with sed, the 2nd run won't give you an
actual editor to change the commit message.

Any ideas, how can I tell `git` that I want to "reword" nth commit right away? Sure,
I am not the first one to stumble upon it, am I? Any ideas?

1: https://github.com/Hi-Angel/dotfiles/blob/0b9418224e4ce7c9783dbc2d9473fd1991b9b0b2/.zshrc#L148-L160