etckeeper is a collection of tools to let /etc be stored in a git, mercurial, or bzr repository. It hooks into apt (and other package managers) to automatically commit changes made to /etc during package upgrades. It tracks file metadata that revison control systems do not normally support, but that is important for /etc, such as the permissions of /etc/shadow. It's quite modular and configurable, while also being simple to use if you understand the basics of working with revision control.