Store changedAttributes instead of attributes

Avoid storing all of the referenced block's attributes in state by
storing only the changed attributes, and name this state property
changedAttributes to reflect this and to avoid ambiguity.

blocks/library/block/index.js

@@ -30,7 +30,7 @@ class ReusableBlockEdit extends Component {
 		this.state = {
 			isEditing: !! ( reusableBlock && reusableBlock.isTemporary ),
 			title: null,
-			attributes: null,
+			changedAttributes: null,
 		};
 	}
 
@@ -41,29 +41,28 @@ class ReusableBlockEdit extends Component {
 	}
 
 	startEditing() {
-		const { reusableBlock, block } = this.props;
+		const { reusableBlock } = this.props;
 
 		this.setState( {
 			isEditing: true,
 			title: reusableBlock.title,
-			attributes: block.attributes,
+			changedAttributes: {},
 		} );
 	}
 
 	stopEditing() {
 		this.setState( {
 			isEditing: false,
 			title: null,
-			attributes: null,
+			changedAttributes: null,
 		} );
 	}
 
 	setAttributes( attributes ) {
 		this.setState( ( prevState ) => {
-			if ( prevState.attributes !== null ) {
-				return { attributes: { ...prevState.attributes, ...attributes } };
+			if ( prevState.changedAttributes !== null ) {
+				return { changedAttributes: { ...prevState.changedAttributes, ...attributes } };
 			}
-			return null;
 		} );
 	}
 
@@ -73,21 +72,21 @@ class ReusableBlockEdit extends Component {
 
 	save() {
 		const { reusableBlock, onUpdateTitle, updateAttributes, block, onSave } = this.props;
-		const { title, attributes } = this.state;
+		const { title, changedAttributes } = this.state;
 
 		if ( title !== reusableBlock.title ) {
 			onUpdateTitle( title );
 		}
 
-		updateAttributes( block.uid, attributes );
+		updateAttributes( block.uid, changedAttributes );
 		onSave();
 
 		this.stopEditing();
 	}
 
 	render() {
 		const { isSelected, reusableBlock, block, isFetching, isSaving } = this.props;
-		const { isEditing, title, attributes } = this.state;
+		const { isEditing, title, changedAttributes } = this.state;
 
 		if ( ! reusableBlock && isFetching ) {
 			return <Placeholder><Spinner /></Placeholder>;
@@ -103,7 +102,7 @@ class ReusableBlockEdit extends Component {
 				isSelected={ isEditing && isSelected }
 				id={ block.uid }
 				name={ block.name }
-				attributes={ attributes !== null ? attributes : block.attributes }
+				attributes={ { ...block.attributes, ...changedAttributes } }
 				setAttributes={ isEditing ? this.setAttributes : noop }
 			/>
 		);