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)
+	}
+}

internal/builder/reuse.go

@@ -616,6 +616,8 @@ func unnamed(named []NamedBuild, module, key string, found CheckBuild) string {
 			return ""
 		}
 	}
-	return "a build is kept under this key, and no check verdict of the walk's pull requests names it with its " +
+	// A build of another pull request, or of a head the forge did not merge, is in no verdict the walk is handed
-		"digests: not taken"
+	// (novox/hq issue 703): said with the check that made it, whose head names which head it was.
+	return "a build is kept under this key, by " + found.Said() + ", and no check verdict of the walk's pull requests " +
+		"at the head the forge merged names it with its digests: not taken"
 }

internal/builder/reuse_test.go

@@ -216,7 +216,8 @@ func TestARecordNoVerdictNamesIsRefusedAndBuilt(t *testing.T) {
 			if got.Reused != nil || len(scratch.took) > 0 || !dockerBuilt(r) {
 				t.Fatalf("a record no verdict names was taken: %+v, copied %v", got.Reused, scratch.took)
 			}
-			if !strings.Contains(got.Why, "no check verdict of the walk's pull requests names it") {
+			if !strings.Contains(got.Why, "no check verdict of the walk's pull requests at the head the forge merged names it") ||
+				!strings.Contains(got.Why, "at head abcdefabcdef") {
 				t.Fatalf("the walk says %q", got.Why)
 			}
 		})