Mark ProcessCommitWorker as idempotent

What does this MR do?

The ProcessCommitWorker processed 1 million "duplicate" jobs the past 7 days, and spent 120 hours on that.

Duplicate jobs are jobs that get scheduled when there is already a job in the queue for the same worker with the same arguments.

If the job was (marked as) idempotent, we would be able to deduplicate those jobs automatically when they get scheduled.

See https://docs.gitlab.com/ee/development/sidekiq_style_guide.html#deduplication

gitlab-com/gl-infra/scalability#282 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content