Add missing index on lfs_objects_projects

Discovered by @abrandl.

See
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13894#note_178141602

db/migrate/20190606034427_add_lfs_object_id_index_to_lfs_objects_projects.rb

+# frozen_string_literal: true
+
+class AddLfsObjectIdIndexToLfsObjectsProjects < ActiveRecord::Migration[5.1]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :lfs_objects_projects, :lfs_object_id
+  end
+
+  def down
+    remove_concurrent_index :lfs_objects_projects, :lfs_object_id
+  end
+end

db/schema.rb

@@ -1770,6 +1770,7 @@ ActiveRecord::Schema.define(version: 20190611161641) do
     t.datetime "created_at"
     t.datetime "updated_at"
     t.integer "repository_type", limit: 2
+    t.index ["lfs_object_id"], name: "index_lfs_objects_projects_on_lfs_object_id", using: :btree
     t.index ["project_id"], name: "index_lfs_objects_projects_on_project_id", using: :btree
   end