AFAIR you can just git clone ../path/to/other/local/repo/.git and it'll use hardlinks, so, basically a copy on write
Also, it’s not really CoW. The immutable object store is hardlinked (since it’s immutable it doesn’t really CoW, but that’s not a very important distinction). But working files are full copies, and so is the rest of .git. With a CoW filesystem you can CoW everything, including node_modules and build artifacts.