Make the second merge check pass: the gate's mesh has a chain of its own; the tests follow delivery 2's meaning (issues 1121, 1108, 1109)

The merge gate raises the snapshot's mesh in a throwaway store and now gives it a chain in memory, since the chain lives only on the bus. A module with no commit is carried by no send (a send is read from the commits on the assignments' keys), so the grants step flags only a module with a commit as newly assigned. The tests that sent a module with no build record one, the merge test waits for the merges' consumer before publishing and for the last look of a window before the next test, the pending test follows a chain that deletes nothing, and a wait's settling is a value of its key.

cmd/mesh-controller/awaiting_push_test.go

@@ -46,7 +46,21 @@ func pushedBy(t *testing.T, open *stores, node string) string {
 		t.Fatal(err)
 	}
 	digest := digestOf(body)
-	if err := open.inventory.RecordSent(ctx, record.ID, digest, declared.Builds); err != nil {
+	// A push records the build of each module it carries (plan.go, Allocating); a module is carried by a send only when it was
+	// built from a commit, for what a machine was sent is read from the commits its assignments name (issue 1109).
+	builds := declared.Builds
+	if builds == nil {
+		current, err := open.inventory.CurrentBuilds(ctx)
+		if err != nil {
+			t.Fatal(err)
+		}
+		names := make([]string, 0, len(plan.Modules))
+		for _, m := range plan.Modules {
+			names = append(names, m.Module)
+		}
+		builds = carriedBuilds(names, nil, current, nil)
+	}
+	if err := open.inventory.RecordSent(ctx, record.ID, digest, builds); err != nil {
 		t.Fatal(err)
 	}
 	return digest