Create parent nodes with the corresponding paths, not with
the absolute path of the child node. Otherwise we will get
incorrect QFileInfo at least in the following case:

    QFileSystemModel model;
    model.setRootPath("/usr/bin");
    QModelIndex idx = model.setRootPath("/usr");
    qDebug() << model.fileInfo(idx).absoluteFilePath();

Without the fix it prints "/usr/bin".

It's a regression triggered by 61cefb2f


(De-inline QFileSystemModel::fileInfo() and implement it efficiently).

Change-Id: I3b4e5f5b256711e27ad50824eaa8492dbc096808
Task-number: QTBUG-51586
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>