Fix commit status API incorrectly overwriting MR head pipeline

The commit status API would sometimes incorrectly overwrite the merge
request head pipeline with a branch pipeline when a merge request
pipeline also existed for the same commit.

This was due to a duplicate and incorrect implementation of the required
logic. The solution is to instead fire a `Ci::PipelineCreatedEvent` _if_
an `external` pipeline is created, as this is the canonical way to
trigger the `head_pipeline_id` update. There is no need to update the
`head_pipeline_id` for pre-existing pipelines, as these would already
have fired the required events.

See https://gitlab.com/gitlab-org/gitlab/-/issues/450176

Changelog: fixed