As sub-tasks are a sub-set of a task that should logically be completed
for the parent task to be completed, it would make sense for the
sub-tasks to share the same milestone as their parent task by default.
The milestone of a parent task is automatically set to its subtasks if:
- The subtask has no milestone set
- AND They belong to the same project or the subtask has no project set
- OR they shared the same milestone before the change on the parent
task (side effect for that case: if an invalide milestone is set on the
parent task, both parent task's and subtasks' milestones which are equal
are gonna be reset)
task-3450281
closesodoo/odoo#130439
Signed-off-by: Xavier Bol (xbo) <xbo@odoo.com>