Feature Request: `git commit --amend-to`

`git commit --amend` is very useful for adding changes to the previous commit quickly.

Unfortunately, it is more difficult to add changes to older commits. To do so I have to enter interactive rebase, mark the commit to change, make the changes, amend the previous commit, and continue the interactive rebase.

It would be nice if there was a single command to do this like `git commit --amend-to SHA`

I have solved this issue for myself with this handy alias I found online ``` git config --global alias.amend-to '!f() { SHA=`git rev-parse "$1"`; git commit --fixup "$SHA" && GIT_SEQUENCE_EDITOR=true git rebase --interactive --autosquash "$SHA^"; }; f' ```

I think it would be beneficial if a command `git commit --amend-to` could be officially added.

Thanks, Aidan Gallagher