Flag `merge_request_diff_files` as a large table in the `AddColumnWithDefaultToLargeTable` cop
As @smcgivern pointed out in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13086, we can't use add_column_with_default
on this table because of its size. We don't currently consider it large for the purposes of the RuboCop, but it's a good thing Sean's disciplined:
gitlabhq_production=# select count(*) from merge_request_diff_files;
count
---------
5860946
(1 row)
So we should probably add it.