Add missing hyphen when computing urls

[jmini]

Fixes #320

Testing done

Submitter checklist

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• Ensure that the pull request title represents the desired changelog entry
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[jmini]

I will test it on my jenkins test installation and add some report here.

[jetersen]

Would be nice to find the changelog for GitLab t document the supported GitLab version.

[jmini]

The issue linked by @ChristopherZellermann indicates that the hyphen was introduced 3 years ago:
https://gitlab.com/gitlab-org/gitlab/-/issues/214217

But now with GitLab 16.0, it seems that the URL without the hyphen are no longer working.

Even more details from:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/112075/diffs#61f0ea3834ef63ce9a722914025e075aad9b3f2d

When subgroups were introduced in GitLab 9.0, a /-/ delimiter was added to URLs to signify the end of a group path. All GitLab URLs now use this delimiter for project, group, and instance level features.

GitLab 16.0 removes legacy URLs from the GitLab application.

(this is probably also published somewhere on the website)

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I just tested with our 15.x installation, and there the redirect was working:
<server>/a-group/a-subgroup/a-poject/merge_requests/135

was redirecting to:
<server>/a-group/a-subgroup/a-poject/-/merge_requests/135

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So I think it is time to adjust this plugin.

[jetersen]

Thanks for info. Thats helps make this easy change even easier :)