WIP: Don't insert "No changes" for a pre-existing version header

This makes Changelog::Updater idempotent. If we perform a release, updating the changelog and removing the data files, then abort for some reason during a later phase, re-running the release process will no longer insert "No changes" directly above a bunch of changes that we just inserted during the previous run.

To facilitate this, Changelog::Updater#insert no longer takes a String of Markdown to insert, but instead takes the MarkdownGenerator object so that we can query its entries property before performing the insert.