X δbtail: tests: Use Clone in verify_rebuild

	$ time go test -failfast -short -run 'ΔBTail/rebuild' >x

before:

	real    1m44,975s
	user    2m18,809s
	sys     0m12,384s

after:

	real    1m15,669s
	user    1m31,340s
	sys     0m10,171s

wcfs/δbtail.go

@@ -449,6 +449,46 @@ func NewΔBtail(at0 zodb.Tid, db *zodb.DB) *ΔBtail {
 	}
 }
 
+// xverifyΔBTail_rebuild needs ΔBTree.clone because otherwise it is too slow to run that test.
+func (orig *ΔBtail) clone() *ΔBtail {
+	klon := NewΔBtail(orig.Tail(), orig.db)
+	// δZtail
+	for _, δZ := range orig.δZtail.Data() {
+		klon.δZtail.Append(δZ.Rev, δZ.Changev)
+	}
+
+	// trackIdx
+	for oid, t := range orig.trackIdx {
+		klon.trackIdx[oid] = &nodeTrack{parent: t.parent, nchild: t.nchild}
+	}
+	// trackNew
+	for oid, t := range orig.trackNew {
+		klon.trackNew[oid] = &nodeTrack{parent: t.parent, nchild: t.nchild}
+	}
+
+	// byRoot
+	for root, origΔTtail := range orig.byRoot {
+		klonΔTtail := &ΔTtail{}
+		klonΔTtail.vδT = append(klonΔTtail.vδT, origΔTtail.vδT...)
+		for k, v := range origΔTtail.KVAtTail {
+			klonΔTtail.KVAtTail[k] = v
+		}
+		for k, rev := range origΔTtail.lastRevOf {
+			klonΔTtail.lastRevOf[k] = rev
+		}
+		klon.byRoot[root] = klonΔTtail
+	}
+
+	// holeIdxByRoot
+	for root, origHoleIdx := range orig.holeIdxByRoot {
+		klonHoleIdx := treeSetKey{SetKey{}}
+		klonHoleIdx.Update(origHoleIdx.SetKey)
+		klon.holeIdxByRoot[root] = klonHoleIdx
+	}
+
+	return klon
+}
+
 // newΔTtail creates new empty ΔTtail object.
 func newΔTtail() *ΔTtail {
 	return &ΔTtail{

wcfs/δbtail_test.go

@@ -912,11 +912,10 @@ func xverifyΔBTail_rebuild(t *testing.T, db *zodb.DB, treeRoot zodb.Oid, t0, t1
 
 					t.Run(fmt.Sprintf(" →%s T%s;R", t2.tree, keys2), func(t *testing.T) {
 
-						// XXX implement Clone and go the clone way if test starts running too slow
-						//δbtail_ := δbtail.Clone()
-						δbtail = NewΔBtail(t0.at, db)
-						step1()
-						δbtail_ := δbtail
+						//δbtail = NewΔBtail(t0.at, db)
+						//step1()
+						//δbtail_ := δbtail
+						δbtail_ := δbtail.clone()
 
 
 						// tracked keys1 becomes tracked keys1_2 after Update(t1->t2)