Refactor the logic of updating head pipelines for merge requests

What does this MR do?

• Use centuralized method MergeRequest#update_head_pipeline
• Filter with sha, for finding the real latest pipeline

This MR is an important refactoring for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23217 Deliverable issue/MRs

Because it addresses the following concerns

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23217#note_121105057
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23217#note_120794027
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23217#note_121001121

What are the relevant issue numbers?

Related: https://gitlab.com/gitlab-org/gitlab-ce/issues/15310

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated
• Tests added for this feature/bug
• 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