ci: move merge-request check into separate stage

The current merge-request job is pretty suboptimal for non-MR branches:

In the gitlab pipeline view, non-MR branches are always shown with warning, since this check always fails if the branch isn't associated with some MR. Thus one always has to check the details page in order to see whether there was something else in that stage (eg. unclean commit messages, etc) or just the always failing merge-request check.

A trivial solution to that problem is just moving that check into a fourth stage, so it's trivial to differenciate in the pipeline list view.

Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net