Bugfix (consider porting): undo triggered

Transaction::cancel was sometimes triggering an undo:
when the current transaction was empty, "commit" did just
remove it which made the subsequent undo act on the
*previous* transaction.

src/db/db/dbManager.cc

@@ -147,15 +147,24 @@ Manager::last_transaction_id () const
 void
 Manager::cancel ()
 {
+  //  equivalent to commit and undo. But takes care that an empty commit is not followed by undo
+  //  (which would undo the previous transaction!)
   if (m_enabled) {
 
-    //  commit and undo - revert changes done so far
-    commit ();
-    undo ();
+    tl_assert (m_opened);
+    tl_assert (! m_replay);
+    m_opened = false;
 
-    //  delete all following transactions
-    erase_transactions (m_current, m_transactions.end ());
-    m_current = m_transactions.end ();
+    if (m_current->first.begin () != m_current->first.end ()) {
+
+      ++m_current;
+      undo ();
+
+    } else {
+      //  empty transactions .. just delete
+      erase_transactions (m_current, m_transactions.end ());
+      m_current = m_transactions.end ();
+    }
 
   }
 }