Default conffile change in hardy -> lucid transition silently stops etckeeper hooks

To reproduce:

1. install Hardy
2. apt-get install etckeeper
3. etckeeper init # will create /etc/.git, since that's the default
4. upgrade to Lucid
5. apt-get install something

Expected behaviour:

* apt-get install runs the apt hooks and adds two commits to the /etc git repository

Actual behaviour:

* nothing happens

Elaboration:

* The default VCS setting in /etc/etckeeper/etckeeper.conf has changed from git to bzr. Since the conffile was not modified, dpkg replaced it with the new version on upgrade. Now etckeeper commit fails to find /etc/.bzr and complains, while etckeeper pre-install/post-install silently do nothing. If a sysadmin doesn't manually invoke 'etckeeper commit' (preferring manual 'git commit'), he may not notice those apt hooks stopped working until he tries to look at 'git log' about a year later.