Simplest I can think of:

    $ echo 'cat > "$@"' >editor
    $ chmod +x editor 
    $ GIT_EDITOR=./editor git commit -a
    stuff
    [master b2d3915] stuff
     1 file changed, 1 insertion(+)

You'll need to hit enter before ^D.

You can add simple Emacs-like keybindings by changing that into

    $ echo 'rlwrap cat > "$@"' >editor

Or just learn ed:

    $ GIT_EDITOR=ed git commit --amend -a 
    256
    1c
    stuff and more stuff
    .
    w
    271
    [master c5092a6] stuff and more stuff
     Date: Thu Apr 6 12:58:18 2017 +0200
     1 file changed, 1 insertion(+)

(the numbers are ed telling me how much was read and written; `1c` means change the first line; `.` means I'm done inserting (go back to command mode) and `w` means write/save; exit with ^D)