Add link to GitLab comparison when a branch lags behind

Point to the GitLab compare tool when a branch is lagging behind the others, to let people find easily the missing commits and potentially create a merge request for them using the handy button in the comparison page itself.

This should let people deal with the Branch apertis/v2022/0.2020.30 lags behind apertis/v2023dev1/0.2020.31 reports in literally two clicks (well, three, to set the MR to automerge). :)