Add clean-up phase for ScheduleDiffFilesDeletion

Process any remaining ScheduleDiffFilesDeletion workers and remove
temporary added index.

changelogs/unreleased/osw-clean-up-phase-for-diff-files-removal.yml

+---
+title: Add clean-up phase for ScheduleDiffFilesDeletion migration
+merge_request: 21734
+author:
+type: other

db/post_migrate/20180913051323_consume_remaining_diff_files_deletion_jobs.rb

+# frozen_string_literal: true
+
+class ConsumeRemainingDiffFilesDeletionJobs < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  MIGRATION = 'ScheduleDiffFilesDeletion'.freeze
+  TMP_INDEX = 'tmp_partial_diff_id_with_files_index'.freeze
+
+  def up
+    # Perform any ongoing background migration that might still be scheduled.
+    Gitlab::BackgroundMigration.steal(MIGRATION)
+
+    remove_concurrent_index_by_name(:merge_request_diffs, TMP_INDEX)
+  end
+
+  def down
+    add_concurrent_index(:merge_request_diffs, :id, where: "(state NOT IN ('without_files', 'empty'))", name: TMP_INDEX)
+  end
+end