gitlab-ci: Stop using manual jobs for merge requests

gitlab-ci: Stop using manual jobs for merge requests

They were causing trouble with Marge Bot: The project settings require
that the pipeline succeeds before a merge request (MR) can be merged,
otherwise Marge doesn't wait for the pipeline to succeed before merging
an MR assigned to her. But Marge can't start manual jobs, so she would
always time out waiting for pipelines with manual jobs.

To avoid this, use these rules:
* Run the pipeline by default for MRs and main project branches changing
  any files affecting it.
* For other MRs, run a single dummy job which always succeeds.
* Don't run any jobs for main project branch changes (e.g. from an MR
  having been merged) not affecting the pipeline.
* Allow jobs to be started manually on branches of forked projects, as
  before.

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3361>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3361>