Limit JH only check on MR pipelines

Related issue: https://jihulab.com/gitlab-cn/gitlab/-/issues/2715

What does this MR do and why?

failed pipeline: https://jihulab.com/gitlab-cn/gitlab/-/pipelines/1908541/

failed job: https://jihulab.com/gitlab-cn/gitlab/-/jobs/10217949

only-jh-check should only check diff between source branch and target branch for pipelines of a MR.

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Before	After

How to set up and validate locally

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MR acceptance checklist

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