X don't track embedded buckets

Reason: see comments.
Prerequisite to fix test_wcfs_watch_2files

wcfs/δbtail.go

@@ -416,7 +416,6 @@ func (δBtail *ΔBtail) Track(key Key, keyPresent bool, path []Node) error { //
 		}
 	}
 
-
 	_, ok := δBtail.byRoot[root]
 	if !ok {
 		δBtail.byRoot[root] = newΔTtail()
@@ -446,13 +445,21 @@ func (tidx trackIndex) AddPath(path []zodb.Oid) {
 		panic("empty path")
 	}
 
+	// don't explicitly keep track of embedded buckets - they all have
+	// InvalidOid, and thus, if kept in tidx, e.g. T/B1:a and another
+	// T/B2:b would lead to InvalidOid having multiple parents.
+	if l >= 2 && path[l-1] == zodb.InvalidOid {
+		path = path[:l-1]
+	}
+
 	parent := zodb.InvalidOid
 	var ptrack   *nodeTrack = nil
 	var track    *nodeTrack	// XXX kill here
 	var oldTrack bool
 	for _, oid := range path {
-		// XXX skip InvalidOid ?
-		// InvalidOid means embedded bucket - e.g. T/B1:a with bucket not having its own oid.
+		if oid == zodb.InvalidOid {
+			panicf("path has node with invalid oid: %v", path)
+		}
 
 		track, oldTrack = tidx[oid]
 		if !oldTrack {
@@ -994,7 +1001,7 @@ func diffT(ctx context.Context, A, B *Tree, δZTC SetOid, trackIdx trackIndex, h
 			for _, ac := range achildren {
 				acOid := ac.node.POid()
 				_, tracked := trackIdx[acOid]
-				if !tracked {
+				if !tracked && /*cannot skip embedded bucket:*/acOid != zodb.InvalidOid {
 					continue
 				}
 

wcfs/δbtail_test.go

@@ -339,11 +339,10 @@ func (rbs RBucketSet) trackIdx(tracked SetKey) trackIndex {
 	trackIdx := trackIndex{}
 	for k := range tracked {
 		kb := rbs.Get(k)
-		// trackIdx records regular buckets or non-empty embedded bucket
-		// ( empty embedded bucket means there is just empty tree node
-		//   and only that empty tree node is recorded in trackIdx )
+		// trackIdx explicitly records only regular buckets.
+		// embedded buckets all have oid=zodb.InvalidOid and would lead to z
 		newNode := false
-		if (kb.oid != zodb.InvalidOid || len(kb.kv) != 0) {
+		if kb.oid != zodb.InvalidOid {
 			track, already := trackIdx[kb.oid]
 			if !already {
 				track = &nodeTrack{parent: kb.parent.oid, nchild: 0}