Add bbm dictionary content to cop cache key

The BackgroundMigration/DictionaryFile relies upon the contents
of the files in db/docs/batched_background_migrations/ as well as
the Ruby source file of the background migration.

Currently, the Rucocop results cache will not be updated if the
contents of the docs folder changes, which leads to false positives
and negatives.

This change includes the contents of the docs folder in the cache
checksum, so any changes in that directory will bust the result cache.

rubocop/batched_background_migrations_dictionary.rb

@@ -21,6 +21,10 @@ def dictionary_data
           }
         end
       end
+
+      def checksum
+        @checksum ||= Digest::SHA256.hexdigest(dictionary_data.to_s)
+      end
     end
 
     def initialize(queued_migration_version)

rubocop/cop/background_migration/dictionary_file.rb

@@ -51,6 +51,10 @@ def on_class(node)
           add_offense(node, message: format(MSG[error_code], msg_params))
         end
 
+        def external_dependency_checksum
+          RuboCop::BatchedBackgroundMigrationsDictionary.checksum
+        end
+
         private
 
         def valid_url?(url)

spec/rubocop/batched_background_migrations_dictionary_spec.rb

@@ -64,4 +64,14 @@
       expect(described_class.new('random').finalize_after).to be_nil
     end
   end
+
+  describe '.checksum' do
+    let(:dictionary_data) { { c: "d", a: "b" } }
+
+    it 'returns a checksum of the dictionary_data' do
+      allow(described_class).to receive(:dictionary_data).and_return(dictionary_data)
+
+      expect(described_class.checksum).to eq(Digest::SHA256.hexdigest(dictionary_data.to_s))
+    end
+  end
 end

spec/rubocop/cop/background_migration/dictionary_file_spec.rb

@@ -227,4 +227,12 @@ def up
       end
     end
   end
+
+  describe '#external_dependency_checksum' do
+    it 'uses the RuboCop::BatchedBackgroundMigrationsDictionary.checksum' do
+      allow(RuboCop::BatchedBackgroundMigrationsDictionary).to receive(:checksum).and_return('aaaaa')
+
+      expect(cop.external_dependency_checksum).to eq('aaaaa')
+    end
+  end
 end