StorageComparer should not wrap sourceStorage with a memory cache if it extends \Drupal\Core\Config\FileStorage

Problem/Motivation

Whilst working on YAML file parsing caching I noticed that \Drupal\Core\Config\FileStorage use FileCache. This means that the MemoryStorage wrap in \Drupal\Core\Config\StorageComparer::__construct() is pointless.

Steps to reproduce

Import config.

Proposed resolution

Check if $source_storage is a FileStorage before wrapping.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet