Daniel was updating the bugzilla at freedesktop.org and pinged me about setting up a git mirror for the bugzilla CVS repository so he could keep track of the customization changes. This makes a lot of sense since bugzilla is probably one of the most customized projects - at any rate, I haven't seen a deployment that hasn't had some tweaking, either bug-buddy integration, work-flow features, changes to make it work with the rest of the site infrastructure or just plain branding.
Whether or not the local changes are to be upstreamed, it makes a lot of sense to track them using git, since git was pretty much designed to handle the bunch-of-local-changes use case. And the git mirror is live, that is, it's updated twice a day, so if you plan to upstream your local changes, it's easy to get the latest upstream work and diff against that.
The git repo is browsable here and can be cloned using the command
git clone git://people.freedesktop.org/~krh/bugzilla
Might be useful.