Label pull requests while the merge queue checks them

Set checking_label under merge_queue to label a pull request while the merge queue is actually checking it. Where queued_label marks every pull request sitting in the queue, checking_label narrows that down to the ones the queue is working on right now: it goes on from the moment a pull request enters a batch and comes off when it leaves the queue, and it stays on while a pull request waits in a bisection, before its own CI has started. No label is added unless you set the option.