ci: fix pipeline run for merge requests

This way CI pipeline runs for branches and tags
and makes it show up in merge requests where
a branch is used as source branch.

Makes all jobs show up in merge request CI
indicator (not just the whitespace job) and
only runs a single CI pipeline, without the
additional detached pipeline.

https://docs.gitlab.com/ce/ci/yaml/#workflowrules-templates



Signed-off-by: Ralph Giles <giles@thaumas.net>