Populate organization_id for merge_request_diff_commit_users table

What does this MR do and why?

• This MR adds the required model changes to MergeRequest::DiffCommitUser model class
• Step 2 of this epic: Shard merge_request_diff_commit_users (&16677 - closed)
• Unblocks:
  • [Non-Backfill Approach] Add a temporary column ... (#536253 - closed)
  • Drop unique index on (name, email) (#526298 - closed)

How to set up and validate locally

• Create a MR and push a commit
• Ensure that new entry gets created in merge_request_diff_commit_users table with the correct organization_id value
• Make sure diff commits are working as expected and new records have non-null value for organization_id

Screenshots

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Related to #517132 (closed)