Ensure `Ci::Pipeline#all_merge_requests` returns a correct merge request list

@ayufan said

all_merge_requests does include current merge request, but if for whatever reason we change the semantic of ref: or source_project: it will stop, maybe we should ensure that condition there is sha IN (?) or merge_request_id=?. It will be silent unnoticed bug. But this as well falls into Finder category.

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The following discussion from gitlab-ce!23217 should be addressed:

• @ayufan started a discussion: (+3 comments)

  Do we miss fix here to #all_merge_requests?