Possible bug: Incorrect counting for Todo

Example:

```
* Todo [0/3]
[ ] Hello

* ABC
[ ] do W

* CDE
X,Y, happened
-> [ ] do Z
```

Behaviour: For `[X] Hello`, the count updates to `[1/3]`. For `[X] do W` or `-> [X] do Z`, the count does not update. I think that the desirable behaviour would be `Todo [0/1]` which updates only when `[ ] Hello` is updated.

No file or test is named. Start by reproducing the issue's Todo example and trace how checkbox lines contribute to the count; done means only the direct Hello checkbox affects Todo [0/1], while do W and do Z do not change it.

Written by the indexing model from the issue text.