QQuickTreeModelAdaptor1 :: lastChildIndex does not consider rootIndex

一个 TreeView 的问题（5.15），折叠一个目录时，没有收起子节点的 View。

排查下来问题出在 QQuickTreeModelAdaptor1::lastChildIndex 中。这个方法没有考虑 rootIndex，到了 root 还没有退出 while 循环。

In English:

 
A TreeView issue (5.15), when collapsing a directory, the sub-node View is not collected.
 
The problem lies in QQuickTreeModelAdaptor1 :: lastChildIndex. This method does not consider rootIndex, and it has not exited the while loop when it reaches root.

 
How to reproduce:
 

• Use FileSystem TreeView demo from Qt
• Create a directory struct:

  test
     - 1
        -  2
            - 3
               - 4
                 - README.txt
     - 5

   

• fsm->setRootPath("D:/test");
• engine.rootContext()>setContextProperty("rootPathIndex", fsm>index(fsm->rootPath() + "/1/2"));
• start program
• expand directory 3 and 4
• fold directory 3 and it does't fold
• expand directory 3 again, it will create duplicate nodes (as the second picture)

See test.zip and filesystembrowser.zip