Keep a check build whose key is unchanged instead of building it again

A fresh check asked when only the trunk moved rebuilt every image, though
the reuse key said nothing it is built from changed: a controller check
took about 12 minutes instead of 5.5. The check now keeps a build kept
under its key when a verdict of the build seat names that key and every
digest, names it as the build's own check so a walk takes it, and the
cleanup keeps it while any check names it (novox/hq issue 703).

internal/link/queue.go

@@ -375,6 +375,26 @@ func ReadCheckBuilds(ctx context.Context, js *broker.JetStream, seat string, pul
 	for _, p := range pulls {
 		wanted[p] = true
 	}
+	return readCheckBuilds(ctx, js, seat, since, func(p PullRef, _ CheckBuilt) bool { return wanted[p] })
+}
+
+// ReadCheckBuildsOf is every build the seat's checks of any pull request said they made, or kept, of those modules
+// since a moment (novox/hq issue 703): what a check may keep instead of building. Newest first.
+func ReadCheckBuildsOf(ctx context.Context, js *broker.JetStream, seat string, modules []string, since time.Time) ([]CheckedBuild, error) {
+	if len(modules) == 0 {
+		return nil, nil
+	}
+	wanted := map[string]bool{}
+	for _, m := range modules {
+		wanted[m] = true
+	}
+	return readCheckBuilds(ctx, js, seat, since, func(_ PullRef, b CheckBuilt) bool { return wanted[b.Module] })
+}
+
+// readCheckBuilds is every build the seat's checks said they made, or kept, that wanted takes. A kept build is named
+// as the check's that built it (novox/hq issue 703): the record under its key is that check's.
+func readCheckBuilds(ctx context.Context, js *broker.JetStream, seat string, since time.Time,
+	wanted func(PullRef, CheckBuilt) bool) ([]CheckedBuild, error) {
 	subject := BuildOutcomeOf(seat)
 	sub, err := js.Context().PullSubscribe(subject, "",
 		nats.BindStream(broker.EventsStream), nats.StartTime(since), nats.AckNone())
@@ -402,12 +422,13 @@ func ReadCheckBuilds(ctx context.Context, js *broker.JetStream, seat string, pul
 				continue
 			}
 			pull := PullRef{Owner: r.Checked.Owner, Repo: r.Checked.Repo, Number: r.Checked.Number}
-			if !wanted[pull] {
-				continue
-			}
 			for _, b := range r.Check.Built {
-				if b.NotBuilt == "" && b.Key != "" {
-					out = append(out, CheckedBuild{Pull: pull, Check: r.ID, Built: b})
+				if b.NotBuilt == "" && b.Key != "" && wanted(pull, b) {
+					check := r.ID
+					if b.Kept != "" {
+						check = b.Kept
+					}
+					out = append(out, CheckedBuild{Pull: pull, Check: check, Built: b})
 				}
 			}
 		}