Say which check made a build the walk does not take, and test the merged heads

A build of another pull request or of a head the forge did not merge is
in no verdict the walk is handed; the walk now names the check that made
it, so its "built:" line says which head it was (novox/hq issue 703).

cmd/mesh-controller/check_builds_test.go

@@ -262,3 +262,18 @@ func forgetWalkVerdicts() {
 	defer walkVerdicts.Unlock()
 	walkVerdicts.read = map[string]verdictsRead{}
 }
+
+// A walk's merged heads are the forge's word for each merge; a merge announced without its head, or a pull request
+// merged at two heads in one walk, has none, and its walk takes no check's build (novox/hq issue 703).
+func TestAWalksMergedHeadsAreTheForgesWord(t *testing.T) {
+	merge := func(number int, head string) inventory.PlanMerge {
+		return inventory.PlanMerge{Repository: "novox/shop", Number: number, Head: head}
+	}
+	p := &inventory.Plan{Delivery: &inventory.PlanDelivery{Merges: []inventory.PlanMerge{
+		merge(7, "aaaaaaaaaaaa1"), merge(8, ""), merge(9, "bbbbbbbbbbbb"), merge(9, "cccccccccccc")}}}
+	heads := mergedHeadsOf(p)
+	shop := func(n int) link.PullRef { return link.PullRef{Owner: "novox", Repo: "shop", Number: n} }
+	if heads[shop(7)] != "aaaaaaaaaaaa1" || heads[shop(8)] != "" || heads[shop(9)] != "" {
+		t.Fatalf("merged heads %v", heads)
+	}
+}