Use `pre-commit` to prevent merging

We don't want to merge branches! We should be using rebase instead. Fast-forward merges are acceptable

Use pre-commit to block merges. I think the `pre-merge-commit` hook is what we want to tie in to https://pre-commit.com/index.html#pre-merge-commit

I don't think this hook will run on fast-foward merges, which is desirable as above

