Add default for external_diff_store

So new records will never have `NULL` external_diff_store, in
preparation for enforcing no `NULL`s.

db/migrate/20200804041018_add_default_value_for_external_diff_store_to_merge_request_diffs.rb

+# frozen_string_literal: true
+
+class AddDefaultValueForExternalDiffStoreToMergeRequestDiffs < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    with_lock_retries do
+      change_column_default :merge_request_diffs, :external_diff_store, 1
+    end
+  end
+
+  def down
+    with_lock_retries do
+      change_column_default :merge_request_diffs, :external_diff_store, nil
+    end
+  end
+end

db/schema_migrations/20200804041018

+ac65340525eba0cd025be706dde4deaf4574f8a7ffb9531d56ffae707e7e8b6d
\ No newline at end of file

db/structure.sql

@@ -13003,7 +13003,7 @@ CREATE TABLE public.merge_request_diffs (
     start_commit_sha character varying,
     commits_count integer,
     external_diff character varying,
-    external_diff_store integer,
+    external_diff_store integer DEFAULT 1,
     stored_externally boolean
 );