I don't think you can add modifier keys in shortcuts.
And this behavior should come out of the box, not me changing stuff around so I can make it usable. For something that I use all the time, sure, but I only use a terminal text editor with git, and I don't use git that often. For everything else, I use a GUI text editor (mousepad, leafpad, whatever).
You can set which editor to use with git using the GIT_EDITOR environment variable instead of telling other people their editor isn't usable by your standards.