use quantity from store, instead of passing in to hook +

+ fix latent bug in useStoreCartItem - the cartItem value is now object:
  - was previously single-item array
  - (note no client code is using this at present)

assets/js/base/hooks/use-store-cart-item.js

@@ -20,16 +20,15 @@ import { useStoreCart } from './use-store-cart';
  * @see https://github.com/woocommerce/woocommerce-gutenberg-products-block/tree/master/src/RestApi/StoreApi
  *
  * @param {string} cartItemKey Key for a cart item.
- * @param {number} initialQuantity Quantity of the cart item.
  * @return {StoreCartItem} An object exposing data and actions relating to cart items.
  */
-export const useStoreCartItem = ( cartItemKey, initialQuantity ) => {
+export const useStoreCartItem = ( cartItemKey ) => {
 	const { cartItems, cartIsLoading } = useStoreCart();
-	const cartItem = cartItems.filter( ( item ) => item.key === cartItemKey );
+	const cartItem = cartItems.find( ( item ) => item.key === cartItemKey );
 
 	// Store quantity in hook state. This is used to keep the UI
 	// updated while server request is updated.
-	const [ quantity, changeQuantity ] = useState( initialQuantity );
+	const [ quantity, changeQuantity ] = useState( cartItem.quantity );
 	const [ debouncedQuantity ] = useDebounce( quantity, 400 );
 
 	const results = useSelect(
@@ -57,7 +56,7 @@ export const useStoreCartItem = ( cartItemKey, initialQuantity ) => {
 
 	// Observe debounced quantity value, fire action to update server when it changes.
 	useEffect( () => {
-		if ( initialQuantity === debouncedQuantity ) return;
+		if ( cartItem.quantity === debouncedQuantity ) return;
 		asyncChangeQuantity( debouncedQuantity );
 	}, [ debouncedQuantity ] );