src/vs/platform/userDataSync/common/extensionsSync.ts/ExtensionsSynchroniser/getPreviewContent
class ExtensionsSynchroniser:	private getPreviewContent(localExtensions: ISyncExtension[], added: ISyncExtension[], updated: ISyncExtension[], removed: IExtensionIdentifier[]): string {
		const preview: ISyncExtension[] = [...added, ...updated];

		const idsOrUUIDs: Set<string> = new Set<string>();
		const addIdentifier = (identifier: IExtensionIdentifier) => {
			idsOrUUIDs.add(identifier.id.toLowerCase());
			if (identifier.uuid) {
				idsOrUUIDs.add(identifier.uuid);
			}
		};
		preview.forEach(({ identifier }) => addIdentifier(identifier));
		removed.forEach(addIdentifier);

		for (const localExtension of localExtensions) {
			if (idsOrUUIDs.has(localExtension.identifier.id.toLowerCase()) || (localExtension.identifier.uuid && idsOrUUIDs.has(localExtension.identifier.uuid))) {
				// skip
				continue;
			}
			preview.push(localExtension);
		}

		return this.stringify(preview, false);
	}