X wcfs: Fix tests to pass again

I broke wcfs test while trying to implement rebuild.
Keep wcfs tests passing while rebuild is not finished yet.

wcfs/internal/xbtree/δbtail.go

@@ -32,6 +32,10 @@ import (
 	"lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xtail"
 )
 
+// XXX malfancitioning rebuild currently breaks wcfs tests
+// TODO kill this after rebuild is finished
+const XXX_killWhenRebuildWorks = true
+
 // ΔBtail represents tail of revisional changes to BTrees.
 //
 // It semantically consists of
@@ -248,6 +252,13 @@ func (δBtail *ΔBtail) Track(key Key, keyPresent bool, path []Node) error { //
 		}
 	}
 
+// XXX hack - until rebuild is implemented
+if XXX_killWhenRebuildWorks {
+	_, ok := δBtail.byRoot[root]
+	if !ok {
+		δBtail.byRoot[root] = newΔTtail()
+	}
+}
 
 	// XXX update diff	XXX here? or as separate step?
 	// XXX update lastRevOf
@@ -396,11 +407,18 @@ func (δBtail *ΔBtail) Update(δZ *zodb.EventCommit) (_ ΔB, err error) {
 	headOld := δBtail.Head()
 	defer xerr.Contextf(&err, "ΔBtail update %s -> %s", headOld, δZ.Tid)
 
+if XXX_killWhenRebuildWorks {
+// XXX hack - until vvv is reenabled
+	δBtail.trackSet.UnionInplace(δBtail.trackNew)
+	δBtail.trackNew = PPTreeSubSet{}
+} else {
+// XXX reenable (currently breaks wcfs tests)
 	// update .trackSet and vδB from .trackNew
 	err = δBtail.rebuild()
 	if err != nil {
 		return ΔB{}, err
 	}
+}
 
 	tracef("Update  @%s -> @%s\n", δBtail.Head(), δZ.Tid)
 	tracef("δZ:\t%v\n", δZ.Changev)