version method
Implementation
int version(String fid) {
final dir = p.join(_root, historyDir, fid);
if (!FileSystemEntity.isDirectorySync(dir)) {
return 0;
}
final items = Directory(dir).listSync();
final ver = items.map((e) => FileSystemEntity.isDirectorySync(e.path) ? 1 : 0)
.reduce((v, e) => v + e);
return ver + 1;
}