Use logic from auto-changelog to get repository URL

[mcmire]

This tool uses the URL of the repository to populate links in the changelog when updating it. The logic to obtain this URL, however, is different from the logic that the auto-changelog executable uses when it validates the changelog. Namely, this tool uses git config --get remote.origin.url, whereas auto-changelog reads it from the package.json (in the case of a Yarn v3 project, anyway). We should make these the same so that create-release-branch does not update the changelog in such a way that makes it invalid.