Look up upstream commits once before queuing ProcessCommitWorkers

What does this MR do?

Resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/65464 by implementing the solution proposed in https://gitlab.com/gitlab-org/gitlab-ce/issues/65464#note_203406659.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content