`on-branch, new-branch, empty-branch` keywords for only/except

Problem to Solve

Clicking the new branch button creates a new branch, which will be picked up by GitLab and will be post processed. In turn, a pipeline will be created.

The pipeline could be useless in cases where there is nothing different from the origin, even considering .gitlab-ci.yml conditions.

Proposal

Adding gitlab-ci keywords like on-branch and new-branch or empty-branch usable in except and only would be very helpful. So no one has to change the default behaviour, but everyone has a change to reduce the required resources and speed up.