88171 r=bienvenu sr=sspitzer; make undo work for multiple mail windows.

mailnews/imap/src/nsImapMailFolder.cpp

@@ -5189,7 +5189,7 @@ nsresult
 nsImapMailFolder::SetTransactionManager(nsITransactionManager* txnMgr)
 {
     nsresult rv = NS_OK;
-    if (txnMgr && !m_transactionManager)
+    if (txnMgr)
         m_transactionManager = txnMgr;
     return rv;
 }

mailnews/local/src/nsLocalMailFolder.cpp

@@ -1508,7 +1508,7 @@ nsresult
 nsMsgLocalMailFolder::SetTransactionManager(nsITransactionManager* txnMgr)
 {
   nsresult rv = NS_OK;
-  if (!mTxnMgr)
+  if (txnMgr)
     mTxnMgr = do_QueryInterface(txnMgr, &rv);
   return rv;
 }