Org Mover: Implement selective sync scope for MergeRequestDiff

Problem

We need to implement selective sync by organization: &17004. We can break up the work by data type.

Proposal

Implement the selective sync scope for the MergeRequestDiff model.

Prerequisites

• merge_request_diffs has a sharding key

If #514251 (closed) is not yet done, then implement a stub for GeoNode.selective_sync_by_organizations? which always returns false)

Acceptance criteria

When selective sync by organization is selected, then merge request diffs will be selectively synced on the secondary site, and selectively checksummed on the primary site.