transaction: only generate file when we actually close the transaction

Before this change, the file were written for every call to `tr.close()`
exposing data to reader far too early.

mercurial/transaction.py

@@ -266,8 +266,8 b' class transaction(object):'
     @active
     def close(self):
         '''commit the transaction'''
-        self._generatefiles()
         if self.count == 1 and self.onclose is not None:
+            self._generatefiles()
             self.onclose()
 
         self.count -= 1