Added a collaboration for a new savepoint model.

Updated an old collaboration to reflect the change.

src/ZODB/collaborations.txt

@@ -28,7 +28,7 @@ Scenario
         # in the changed state.
     T.commit()
         C.beforeCompletion(T)
-        C.tpc_begin(T, False)
+        C.tpc_begin(T)
             S.tpc_begin(T)
         C.commit(T)
             S.store(1, ..., T)
@@ -89,3 +89,84 @@ Scenario
             C._flush_invalidations()
             # Processes invalidations that may have come in from other
             # transactions.
+
+
+Participants:
+  T: ITransaction
+  o1, o2, o3: some persistent objects
+  C1, C2, C3: resource managers
+  S1, S2: Transaction savepoint objects
+  s11, s21, s22: resource-manager savepoints
+
+Scenario
+    """Rollback of a savepoint"""
+
+        create T
+        o1.modify()
+            C1.regisiter(o1)
+                T.join(C1)
+        T.savepoint()
+            C1.savepoint()
+                return s11
+            return S1 = Savepoint(T, [r11])
+        o1.modify()
+            C1.regisiter(o1)
+        o2.modify()
+            C2.regisiter(o2)
+                T.join(C2)
+        T.savepoint()
+            C1.savepoint()
+                return s21
+            C2.savepoint()
+                return s22
+            return S2 = Savepoint(T, [r21, r22])
+        o3.modify()
+            C3.regisiter(o3)
+                T.join(C3)
+        S1.rollback()
+            S2.rollback()
+                T.discard()
+                    C1.discard()
+                    C2.discard()
+                    C3.discard()
+                        o3.invalidate()
+            S2.discard()
+                s21.discard() # roll back changes since previous, which is r11
+                    C1.discard(s21)
+                        o1.invalidate()
+                        # truncates temporary storage to s21's position
+                s22.discard() # roll back changes since previous, which is r11
+                    C1.discard(s22)
+                        o2.invalidate()
+                        # truncates temporary storage to beginning, because
+                        # s22 was the first savepoint.  (Perhaps conection
+                        # savepoints record the log position before the 
+                        # data were written, which is 0 in this case. 
+        T.commit()
+            C1.beforeCompletion(T)
+            C2.beforeCompletion(T)
+            C3.beforeCompletion(T)
+            C1.tpc_begin(T)
+                S1.tpc_begin(T)
+            C2.tpc_begin(T)
+            C3.tpc_begin(T)
+            C1.commit(T)
+                S1.store(1, ..., T)
+            C2.commit(T)
+            C3.commit(T)
+            C1.tpc_vote(T)
+                S1.tpc_vote(T)
+            C2.tpc_vote(T)
+            C3.tpc_vote(T)
+            C1.tpc_finish(T)
+                S1.tpc_finish(T, f) # f is a callback function, which arranges
+                                   c# to call DB.invalidate (next)
+                    DB.invalidate(tid, {1: 1}, C)
+            TM.free(T)
+            C1.afterCompletion(T)
+                C1._flush_invalidations()
+            C2.afterCompletion(T)
+                C2._flush_invalidations()
+            C3.afterCompletion(T)
+                C3._flush_invalidations()
+