Resolve "rspec spec/lib/gitlab/background_migration/deserialize_merge_request_diffs_and_commits_spec.rb spec/services/system_note_service_spec.rb fails"

If the schema changes after 20171114162227 for any of these models, and specs after this one use factories, then those factories will use the models with outdated column information cached.

We shouldn't really use factories in migration specs, but this is a special case because there is a lot of git-related setup code in the model that would be painful to copy to the migration. Instead, we just manually reset the column information for the models we could pollute.

Closes #42074 (closed).