I was happily able to repurpose some code from Git.Filename to handle this.
I remember writing that code... a whole afternoon at a coffee shop, after
which I felt I'd struggled with Haskell and git, and sorta lost, in needing
to write this nasty peice of code. But was also pleased at the use of a
pair of functions and quickcheck that allowed me to get it 100% right.
So, turns out I not only got it right, but the code wasn't as special-purpose
as I'd feared. Yay!
