A merge on the forge builds what it moved, bases first

The controller follows the forge's merges (novox/hq 04-ISSUES/131). For each
module recorded as built from that repository and branch it records the move to
the merge commit and builds it — bases first, because a module built before the
module it stands on is built against the old one and reports success, and a base
that fails stops what stands on it. Nothing is pushed here: what a finished build
does to the machines running the module stays the upgrade's decision.

Two more things the same ordering gives: `build --behind` builds bases first, and
`build --on <module>` rebuilds everything that stands on a module — the rebuild a
changed base needs, which "behind" does not see because their sources did not
move.

internal/link/serve.go

@@ -58,6 +58,8 @@ type Upgrader interface {
 	// stop every upgrade behind it — except the store unreachable for the moment, which is asked
 	// again for a bounded time (novox/hq issue 083).
 	Upgraded(ctx context.Context, u Upgraded) error
+	// SourceMoved is a merge on the forge: build what that source produces, bases first.
+	SourceMoved(ctx context.Context, m SourceMoved) error
 }
 
 // Server acts on what nodes and modules say.
@@ -101,6 +103,9 @@ func (s *Server) Records(r Recorder) { s.recorder = r }
 
 // Follows says what to do about upgrades, and asks for them to be delivered.
 func (s *Server) Follows(u Upgrader) error {
+	if err := s.inbound.Also(KindSourceMoved); err != nil {
+		return err
+	}
 	if err := s.inbound.Also(KindModuleMoved); err != nil {
 		return err
 	}
@@ -232,6 +237,8 @@ func (s *Server) act(ctx context.Context, m Control) {
 		s.wasBuilt(ctx, m)
 	case KindModuleMoved:
 		s.moved(ctx, m)
+	case KindSourceMoved:
+		s.sourceMoved(ctx, m)
 	case KindCatchUp:
 		s.catchingUp(ctx, m)
 	default:
@@ -603,3 +610,24 @@ func short(commit string) string {
 	}
 	return commit
 }
+
+// sourceMoved acts on the forge's announcement of a merge. Taken whatever happens: a build that
+// fails is reported by the build itself, and re-delivering the merge would only re-fail it.
+func (s *Server) sourceMoved(ctx context.Context, m Control) {
+	var moved SourceMoved
+	if err := json.Unmarshal(m.Body(), &moved); err != nil {
+		s.log.Printf("a merge announcement could not be read: %v", err)
+		_ = m.Took()
+		return
+	}
+	m.About("merge " + moved.Owner + "/" + moved.Repo + " into " + moved.Base)
+	if moved.Commit == "" || moved.Repo == "" {
+		s.log.Printf("a merge announcement named no repository or no commit; ignored")
+		_ = m.Took()
+		return
+	}
+	if err := s.upgrader.SourceMoved(ctx, moved); err != nil {
+		s.log.Printf("%s/%s moved to %.8s and the mesh could not act on it: %v", moved.Owner, moved.Repo, moved.Commit, err)
+	}
+	_ = m.Took()
+}