Column merge_request_diff_commit_users.organization_id should be ready for Cells 1.0

Column merge_request_diff_commit_users.organization_id should meet all of the following requirements for Cell 1.0:

• has no default
• is not nullable
• has a foreign key to organizations

Other spec update:

• remove merge_request_diff_commit_users table from work_in_progress hash: https://gitlab.com/gitlab-org/gitlab/-/blob/master/spec/lib/gitlab/database/sharding_key_spec.rb