Fix for JENKINS-18328

Fix the defective file link for GitLab browsers:

For older GitLab versions, there was a duplicate slash and for
newer versions the link is completely wrong. Fix both:

Old:   http://urlbase//0123456789...01234567/tree/file.name
<5.2:  http://urlbase/0123456789...01234567/tree/file.name
>=5.2: http://urlbase/blob/0123456789...01234567/file.name

Note: I do not know at which version of GitLab the new link
      format was introduced. This patch assumes 5.2.0, but it
      may be that older versions already need the new format.

Closes #JENKINS-18328