I’ve been waiting since November for this.
Now, if you submit a branch for review on Launchpad, Launchpad will automatically generate the diff for you. For example, take a look at the review page for one of lifeless’s subunit branches.
To take advantage of the feature, go to the page of a branch you want to land (you can use bzr lp-open to do this), then click on “Propose for merging into another branch”. Select the branch you want to land it on (normally trunk, which is the default branch), then type some stuff explaining what the patch does. Hit “Submit”, wait a minute or so, then refresh. Voila.
We hope to tweak the page later so that it’s obvious that a diff is being generated and so you don’t have to refresh. Even so, I’m very pleased that we got this done. Thanks particularly to Aaron for getting this landed.