Hi, nim-nim. Patch tracking is different from just having the distro in a version control system. Patch tracking is about discovering what patches have been contributed to solve particular bugs, and about getting those patches to the right places -- the "right place" often being upstream (where "upstream" means the point of origin for the code in question).
For example, one result of a recent Ubuntu Developers Summit was the Launchpad Upstream Improvements spec , whose rationale is: "Upstreams frequently complain that it is difficult for them to find Ubuntu patches. patches.ubuntu.com is a diff between Ubuntu and Debian, so it's not really useful to some upstreams."
In other words, upstreams want to more easily find patches attached to Ubuntu bugs, since sometimes those patches are upstreamable even if the bug report itself has not been linked (formally or informally) to the upstream tracker. One result of this is the "patch tracking" feature coming in Launchpad . I think after reading  and  you'll understand how patch tracking is about more than putting the distribution in a version control system. ( links to some screenshots of the feature, by the way.)