Improve the changelog generation workflow

Using `fixup!` is not ideal, because when squashing with `fixup!`, git expects the rest of the commit summary after `fixup!` to match a previous commit.

Ideally:

- It should be easy for contributors to mark a change as not needing a changelog entry.

- It should be hard for me to accidentally merge commits which don't have changelog entries, since those commits should be squashed .