Fix duplicate merge request pipelines created by Sidekiq worker retry

What does this MR do?

Currently, MergeRequest#merge_request_pipeline_exists? has several problems.

• The name. We changed the terminology about merge request pipelines, and it's still referring to the old terminology.
• It cannot check if (attached) merge request pipelines exist or not. Because, merge request pipelines stores merge-sha (Generated by MergeToRefService) into sha column. Thus diff_head_sha never matches those shas. We have to use for_sha_or_source_sha in this case.
• We thrive reducing the number of scopes for "Find pipelines for merge requests" in MergeRequest model, and having a dedicated method for compatible any cases. This is good from two PoV 1) We can easily maintain 2) Reduce the number of mistakes from developers using a wrong method.

Related: https://gitlab.com/gitlab-org/gitlab-ee/issues/9186

• CE: Here
• EE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/12212

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/7380

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• [-] Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer