Keep a try off the forge and bound how many run (hq issue 686)

Review of #314: a try's start named its commit, which the forge's holder
matches to an open pull request's head and marks pending. Its start now
names no repository or commit. One try per commit and four in all, a try
never ages ahead of deliveries or pull request checks, and its id is
checked before it is a bus subject.

internal/link/queue_order.go

@@ -26,6 +26,8 @@ import (
 //   - a check that has waited CheckWaitsAtMost goes before every delivery asked after it, so no check
 //     waits for ever behind a stream of deliveries;
 //   - within a kind, first come, first served, by when it was asked;
+//   - a branch tried before its pull request goes after everything else, however long it has waited: it is
+//     never promoted ahead of a delivery, and never ahead of a pull request's check (novox/hq issue 686);
 //   - an ask whose kind cannot be told counts as a check: a delivery taken late breaks a budget, a check
 //     taken late breaks nothing.
 //
@@ -64,8 +66,12 @@ func (a QueuedAsk) askedOrStored() time.Time {
 // QueueOrder is the order the waiting asks are to be handed out in, now: the deliveries and every check
 // that has waited CheckWaitsAtMost, by when each was asked, then the other checks, by when each was asked.
 func QueueOrder(waiting []QueuedAsk, now time.Time) []QueuedAsk {
-	var first, then []QueuedAsk
+	var first, then, tries []QueuedAsk
 	for _, a := range waiting {
+		if IsTry(a.Request) {
+			tries = append(tries, a)
+			continue
+		}
 		if KindOf(a.Request) == KindCheck && now.Sub(a.askedOrStored()) < CheckWaitsAtMost {
 			then = append(then, a)
 			continue
@@ -82,7 +88,8 @@ func QueueOrder(waiting []QueuedAsk, now time.Time) []QueuedAsk {
 	}
 	byAsking(first)
 	byAsking(then)
-	return append(first, then...)
+	byAsking(tries)
+	return append(append(first, then...), tries...)
 }
 
 // toMove is the asks to move to the tail, in this order, for the queue to stand as order says: everything