Issues in calculating summary tasks completion

[dbarashev]

Topic on the support portal:
https://help.ganttproject.biz/t/task-completion-calculation/1461

The issue is that completion percentage algorithm in GP 2.8.X takes into account task lengths for all tasks, including summary tasks. That's why the completion of a summary task will be different when e.g. two child tasks are chained with a dependency and when they are not (and go in parallel)

This is fixed in the master branch by #1455 but remains in GP 2.8.X.
We'll probably need project open diagnostic code which reports if completion changes with the new algorithm or not.