You could create a postprocessing script that fetches the committed file contents (via the GitHub API) and compares it to the new file contents. The filename is passed as the first argument (Deno.args[0]), which can be used to get both of those paths.
https://github.com/githubocto/flat#postprocessing

Alternatively, you could also create another step in your workflow that looks at the last commit's diff and works from there.

This is something I'm interested in to. I was thinking about doing it via the git diff, but all I need is to seed the changes into a in-memory db