transaction: use the proper variable in '_addbackupentry' (issue4684)

The 'file' variable was undefined but resolved to the 'file' built-in.
This is why pylint complains about overwriting built-ins...

mercurial/transaction.py

@@ -222,7 +222,7 b' class transaction(object):'
     def _addbackupentry(self, entry):
         """register a new backup entry and write it to disk"""
         self._backupentries.append(entry)
-        self._backupmap[file] = len(self._backupentries) - 1
+        self._backupmap[entry] = len(self._backupentries) - 1
         self._backupsfile.write("%s\0%s\0%s\0%d\n" % entry)
         self._backupsfile.flush()