We document that a commit fixing an issue tracked in GitLab
should put just "Fixes: #NNN" into its commit message. But when
viewing git log, having full URL which is directly clickable is
more developer friendly and GitLab is capable of handling both.
Therefore, document that users should put full URL, just like
when fixing a bug tracked in other sites.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>