transaction: use `self.journal` to create backup file

Transaction journal name is "journal" in most case, but it can be something else.
We use the appropriate attribute to create the file.

mercurial/transaction.py

@@ -154,7 +154,7 b' class transaction(object):'
 
         if file in self.map or file in self.backupmap:
             return
-        backupfile = "journal.%s" % file
+        backupfile = "%s.%s" % (self.journal, file)
         if self.opener.exists(file):
             filepath = self.opener.join(file)
             backuppath = self.opener.join(backupfile)