Keep the builds of the grants step's machine alone, not of every machine its composition resolves (repo-check of #230)

Composing the bus's machine resolves the others on the same context, and
a machine never recorded as sent refused the whole composition.

cmd/mesh-controller/grants_step_test.go

@@ -43,7 +43,7 @@ func sendsRecorded(t *testing.T, holder string, behind bool, refuse error) *[]aS
 		return holder, behind, nil
 	}
 	sendRollout = func(ctx context.Context, _ *stores, names []string) ([]string, error) {
-		s := aSend{names: append([]string(nil), names...), keepsAll: everyBuildKept(ctx)}
+		s := aSend{names: append([]string(nil), names...), keepsAll: len(names) == 1 && everyBuildKept(ctx, names[0])}
 		for n := range scopeOf(ctx).judged {
 			s.judged = append(s.judged, n)
 		}
@@ -168,6 +168,15 @@ func TestOnlyTheUserListsContentIsExempt(t *testing.T) {
 	}
 }
 
+// The step keeps the builds of the machine it sends alone: the others its composition resolves are composed
+// as any send composes them.
+func TestTheGrantsStepKeepsOneMachinesBuilds(t *testing.T) {
+	ctx := keepingEveryBuild(t.Context(), "novox")
+	if !everyBuildKept(ctx, "novox") || everyBuildKept(ctx, "ace") || everyBuildKept(t.Context(), "novox") {
+		t.Error("the grants step keeps the builds of a machine it does not send")
+	}
+}
+
 // The delivery plan says the step before the merge.
 func TestAChangePlanSaysTheGrantsStep(t *testing.T) {
 	const catalogue_ = "http://forge.internal:20000/novox/mesh-catalog.git"