Feat: Git Diff should return the diff between two commits

This is a significantly more advanced feature than what we have already built
and it's one of the _reasons_ we wanted to have this package _separate_ from the App that uses it;
it needs to be _extensively_ independently tested.

We will need to map this out carefully before diving in.
1. Diff a specific file or return a list of all the files with the diff nested?
2. Diff the current branch against the `main` (or `master`) branch?
3. Run the diff on `localhost` as there is no REST API endpoint for this.
4. Parse the result and return as plaintext so that we can visualise it