Say which modules a walk left unchanged, so a merge that moved nothing is not read as stranded (hq issue 611)

A delivery plan made while another walk built a module reads it whole and names it; the walk, cut later,
finds none of the changed files in the build source that build recorded and moves nothing. Its record named
no module, so mesh-delivery read 'matched what runs' as 'never built', stopped the delivery and asked the
operator about a change already on every machine. The walk now says each such module in delivery.unchanged,
with why; its modules keep meaning what it builds.

cmd/mesh-controller/batches.go

@@ -939,11 +939,16 @@ func planBatch(ctx context.Context, open *stores, batch *inventory.Plan, carry [
 	commits, named := carriedOf(merges)
 	var moved []string
 	var members []batchMember
+	unchanged := map[string]string{}
 	for _, m := range combinedMerges(merges) {
 		// **News, whatever was seen since** (review of this change): each merge was judged when it was heard, and
 		// a cut, a fold or a failed walk since marks its modules seen — so a merge walked alone after a failed
 		// walk, or folded, would read as history and move nothing. When it was made is not asked again here.
 		m.MergedAt = ""
+		// **Unchanged, said** (novox/hq issue 611): read before the merge is acted on, from the same records.
+		for n, why := range unchangedOfMerge(m, entries, read) {
+			unchanged[n] = why
+		}
 		names, err := movesOfMerge(ctx, inv, m, entries, read)
 		if err != nil {
 			return err
@@ -995,12 +1000,16 @@ func planBatch(ctx context.Context, open *stores, batch *inventory.Plan, carry [
 	plan.Delivery.Merges = named
 	plan.Delivery.Alone = batch.Delivery != nil && batch.Delivery.Alone
 	plan.Delivery.Withheld = withheld
+	plan.Delivery.Unchanged = unchangedLeft(unchanged, plan.Modules)
 	// **Its moments and its class** (novox/hq ADR 0282 decision 6): measured, never acted on.
 	plan.Times = walkTimesAtCut(*batch, plan, entries, now)
 	if len(moved) == 0 {
 		plan.State = inventory.PlanDone
 		plan.Tiers = [][]string{}
 		plan.Note = "moved nothing the mesh holds: nothing to walk"
+		if len(plan.Delivery.Unchanged) > 0 {
+			plan.Note = "moved nothing the mesh holds: nothing to walk; " + unchangedSaid(plan.Delivery.Unchanged)
+		}
 		if err := inv.SavePlan(ctx, &plan); err != nil {
 			return err
 		}
@@ -1045,6 +1054,31 @@ func planBatch(ctx context.Context, open *stores, batch *inventory.Plan, carry [
 	return nil
 }
 
+// unchangedSaid names the modules a walk says matched what runs, for its note.
+func unchangedSaid(unchanged map[string]string) string {
+	names := make([]string, 0, len(unchanged))
+	for n := range unchanged {
+		names = append(names, n)
+	}
+	sort.Strings(names)
+	return "unchanged, the build the mesh holds is theirs: " + strings.Join(names, ", ")
+}
+
+// unchangedLeft is what a walk says matched what runs: every module a merge left unchanged that the walk does not
+// build anyway, as another merge's module or a dependent (novox/hq issue 611). Nil when none is.
+func unchangedLeft(unchanged map[string]string, builds map[string]*inventory.PlanModule) map[string]string {
+	out := map[string]string{}
+	for n, why := range unchanged {
+		if _, built := builds[n]; !built {
+			out[n] = why
+		}
+	}
+	if len(out) == 0 {
+		return nil
+	}
+	return out
+}
+
 // withhold takes from a plan every module named that its merges do not move, and the tiers left empty: the
 // modules taken.
 func withhold(plan *inventory.Plan, names, moved []string) []string {