src/vs/platform/backup/electron-main/backupMainService.ts/BackupMainService/registerFolderBackupSync
class BackupMainService:	registerFolderBackupSync(folderUri: URI): string {
		if (!this.folders.some(folder => this.backupUriComparer.isEqual(folderUri, folder))) {
			this.folders.push(folderUri);
			this.saveSync();
		}

		return this.getBackupPath(this.getFolderHash(folderUri));
	}