Fix a small race condition on MR pipelines guard

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The following discussion from !23217 (merged) should be addressed:

• @ayufan started a discussion:

  This has slight race-condition:

  1. The merge_request_pipeline_exists? gonna return false, for diff_head_sha=A,
  2. The diff_head_sha gonna be updated to B,
  3. The another call (concurrent) will make the merge_request_pipeline_exists? to return false,
  4. Two CreatePipelineServices gonna be executed.

  I think that we should:

  1. Extend the create_merge_request_pipeline with sha,
  2. We gonna pass merge_request.diff_head_sha to this method call,
  3. Run return if merge_request.merge_request_pipeline_exists?(sha),
  4. To Ci::CreatePipelineService pass ref: merge_request.source_branch, sha: sha.

  Feel free to resolve that as extra MR.