CI: Check: integration branch may have multiple commits

As all projects are commited in the same branch the 'up-to-date' check
may not only check if the first parent commit points to the
master/dunfell branch. Now it is needed to loop through the history
until the integration branch's commit is found.

On fail a message is displayed which merge request needs to be
retriggered manually. This can now also be the 'parent'-MR that
triggered the complete chain.

The check job is used pipeline again.

The retrigger job also looks in the .gitlab-ci project for check jobs to
retrigger.