End the check of a pull request's old head when a new head is checked

A check judges one head; once the pull request has another, its verdict
answers nobody and its run holds a build machine the new head waits
behind. Asking a check of R#N at H now drops every waiting check of R#N
at another head and kills every one in flight, saying each in the log.
Announcements are unordered and a recheck asks a recorded head, so this
happens only when the forge confirms H is the pull request's current
head; otherwise nothing ends and that is said. A one-shot recheck ends
them within the call. Tries, walk builds, group checks, other pull
requests' checks and rechecks of the same head are left alone.

novox/hq issue 726

cmd/mesh-controller/delivery.go

@@ -952,7 +952,7 @@ func deliveryCommand(ctx context.Context, args []string) error {
 		if *group == "" && len(members) == 1 {
 			// One head, checked again as the forge's announcement would have it (a recheck): the controller's
 			// own path for a pull request, run because the delivery's owner asked.
-			if err := sayingOnTheBus(ctx, inv, func() error { return (following{open}).PullUpdated(ctx, members[0].pull()) }); err != nil {
+			if err := sayingOnTheBus(ctx, inv, func() error { return (following{open: open, oneShot: true}).PullUpdated(ctx, members[0].pull()) }); err != nil {
 				return err
 			}
 			return answer(map[string]any{"rechecked": members[0].ID})