> test_sum is assumedly relatively trivial. So as a lay person I’d expect some sort of obviousness test to apply. Like so much of the stuff in the Google/Oracle lawsuit.

Feel free to extrapolate to the threshold where it's not and at that point apply.

> you’re really having to duplicate the actual git internals

Copyright covers the expression, not the method. So the Rust function:

    fn sum(a: u8, b: u8) {
        a + b
    }

    int sum(int a, int b) 
    {
        return a + b;
    }

That's not copyrightable because it's trivial.