gitlab portgroup does not support release downloads

[ryandesign (Ryan Carsten Schmidt)]

The github portgroup on which the gitlab portgroup was based supports setting github.tarball_from releases to download a release tarball; the gitlab portgroup does not offer this.

Perhaps one reason why is that the gitlab portgroup uses use_bzip2 yes in gitlab.setup. Because use_bzip2 cannot be undone, ports using this portgroup can only download .tar.bz2 files. That's quite inconvenient if one wants to download a .tar.xz or .tar.gz release file.