Update Gitlab compare URL for Gitlab v16

[xurizaemon]

From Gitlab v16, compare URLs have the new hypen-slash /-/ path prefix at end of the project path.

Gitlab apparently handles this with a redirect for public repos, but it seems that private repos won't redirect.

Gitlab release notes advise "Update any scripts or bookmarks that reference the legacy URLs."

• https://docs.gitlab.com/ee/update/deprecations.html?removal_milestone=16.0#legacy-urls-replaced-or-removed
• https://gitlab.com/gitlab-org/gitlab/-/issues/28848#release-notes
• https://gitlab.com/pmjones/fake/compare/0.0.1...0.2.0 before (now a redirect)
• https://gitlab.com/pmjones/fake/-/compare/0.0.1...0.2.0 after

NB: I ran make test, it shows what I think is correct output (pmjones/fake "after" URL above), no test updates appear to be required for changed output.

[davidrjonas]

Thank you! I'll cut a new release soon.