改进事务方法，支持嵌套事务提交

ThinkPHP/Library/Think/Db/Driver.class.php

@@ -315,14 +315,16 @@ public function commit()
      */
     public function rollback()
     {
-        if ($this->transTimes > 0) {
+        if ($this->transTimes == 1) {
             $result = $this->_linkID->rollback();
             $this->transTimes = 0;
             $this->transPdo = null;
             if (!$result) {
                 $this->error();
                 return false;
             }
+        } else {
+            $this->transTimes = $this->transTimes <= 0 ? 0 : $this->transTimes-1;
         }
         return true;
     }

ThinkPHP/Library/Think/Model.class.php

@@ -1673,7 +1673,6 @@ public function getTableName()
      */
     public function startTrans()
     {
-        $this->commit();
         $this->db->startTrans();
         return;
     }