cot verification: a decision task shouldn't add its decision task to the chain

[escapewindow]

In bug 1338933, I retriggered a decision task.

The original decision task had a matching taskId and taskGroupId, because it was created through cron.yml.

The retriggered decision task had mismatched taskId and taskGroupId, because it was retriggered with the taskGroupId from the original decision task.

However, if the retriggered decision task is otherwise well-formed and pointing at a valid tree, we shouldn't need to trace it back to the original decision task.

I'm thinking we should put in an exception to the taskGroupId rule: if a task is a decision task, don't trace it back to its original decision task. This assumes that all decision tasks are traceable back to the tree and mach taskgraph currently. If that changes, then we'll have to revisit this rule.