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).

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 " +
-		"digests: not taken"
+	// A build of another pull request, or of a head the forge did not merge, is in no verdict the walk is handed
+	// (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)
 			}
 		})