[FIXED JENKINS-18328] Defective file link for GitLab browsers

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
\<5.1: http://urlbase/0123456789...01234567/tree/file.name
\>=5.1: http://urlbase/blob/0123456789...01234567/file.name

Note: This supersedes pull request #157, which switched over at too high a version number.