Handle gitaly calls during transactions in code review workers

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Overview

We identified some long idle-in-transaction that are stressing our Postgres connection pools, leaving less space for actual traffic.

We identified the following workers as belonging to the groupcode review group:

• app/workers/create_note_diff_file_worker.rb
• app/workers/delete_diff_files_worker.rb
• app/workers/merge_request_cleanup_refs_worker.rb
• app/workers/merge_request_mergeability_check_worker.rb
• app/workers/merge_requests/mergeability_check_batch_worker.rb
• app/workers/merge_requests/update_head_pipeline_worker.rb
• app/workers/merge_worker.rb
• app/workers/migrate_external_diffs_worker.rb
• app/workers/new_merge_request_worker.rb
• app/workers/stuck_merge_jobs_worker.rb

These workers are executing gitaly calls while keeping DB transactions open.

source: !186758 (comment 2518140632)

Deliverables

The goal is to not call gitaly from within an open DB transaction, when possible.

• For each worker in the list, move gitaly calls outside database transactions to reduce database pressure.