go/zodb: ΔTail += Data()

This gives access to raw data stored in ΔTail. We'll need this in the
next commit, as well as going through raw δtail data is also generally
useful.

go/zodb/δtail.go

@@ -87,6 +87,13 @@ func (δtail *ΔTail) Len() int {
 	return len(δtail.tailv)
 }
 
+// Data returns slice with δtail data.
+//
+// The caller must not modify returned slice.
+func (δtail *ΔTail) Data() /*readonly*/ []ΔRevEntry {
+	return δtail.tailv
+}
+
 // Head returns newest database state for which δtail has history coverage.
 //
 // Head is ↑ on Append, in particular it does not ↓ on Forget even if δtail becomes empty.

go/zodb/δtail_test.go

@@ -61,6 +61,10 @@ func TestΔTail(t *testing.T) {
 			t.Fatalf("tailv:\nhave: %v\nwant: %v", δtail.tailv, tailv)
 		}
 
+		if d := δtail.Data(); !tailvEqual(d, tailv) {
+			t.Fatalf("Data():\nhave: %v\nwant: %v", d, tailv)
+		}
+
 		if l := δtail.Len(); l != len(tailv) {
 			t.Fatalf("Len() -> %d  ; want %d", l, len(tailv))
 		}