This method is a copy of getDirectoryContent(OCFile), but receives a parentId instead of an OCFile to get its id with getId().

1. Both methods should have the same name: getFilesByParent or getDirectoryContent, doesn't matter, but both methods the same since do exactly the same.

2. Easy refactoring: let all the code in the method with parameter 'long parentId', and change the other so that it just calls the former passiing OCFile.getFilleId() as parameter.