How to build a `git diff` driver

Something I've been meaning to write about since November 2024 is how to create an external command for diffing between files with git diff. I found that while I was implementing renovate-packagedata-diff that there seemed to be a lack of documentation around how to do it, so it would be worthwhile me blogging about it. (I've since found that there is some documentation in the Git Diffs man page, but it wasn't exactly well discoverable) It's been at the back of my mind as a TODO for some time...