transaction: extract backupentry registration in a dedicated function

We are about to use the 'backupentry' mechanism to allow cleaning up
transaction-related temporary files (such as 'changelog.i.a'). We start
by extracting the entry registration into its own method for easy reuse.

At that point, I would like to rename the backup-file related variable to
something generic but I'm a bit short of ideas.

mercurial/transaction.py

@@ -190,9 +190,13 b' class transaction(object):'
         else:
             backupfile = ''
 
-        self._backupentries.append((file, backupfile))
+        self._addbackupentry((file, backupfile))
+
+    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._backupsfile.write("%s\0%s\n" % (file, backupfile))
+        self._backupsfile.write("%s\0%s\n" % entry)
         self._backupsfile.flush()
 
     @active