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.)
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