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/release.go

@@ -1589,7 +1589,9 @@ func grantsOnlyIn(ctx context.Context, open *stores, holder string, plan catalog
 	var other []string
 	listID := ""
 	for _, m := range plan.Modules {
-		if _, was := sent[m.Module]; !was {
+		// A module no build is recorded for is carried with no commit and names none on its key (issue 1109), so it is "sent"
+		// only by the comparison of the summaries below; a module with a commit that was never sent is a new assignment.
+		if _, was := sent[m.Module]; !was && declared.Builds[m.Module] != "" {
 			other = append(other, m.Module+" newly assigned")
 		}
 		if m.BusUsers != "" && m.ClaimsSeat(catalogue.BrokerSeat) {