empty diff shown for some merged proposals

[Martin Pool]

If a branch is proposed and then soon afterwards merged into the target, you can end up with a merge proposal whose web page shows no diff, or an empty or nonsensical diff.

<https://code.launchpad.net/~wgrant/pydkim/split-out-crypto/+merge/52776> is one instance; I've seen others.

What should happen instead is that the diff should be generated between the tip of the source branch, and the target revision immediately before it was merged. Then the mp page will be a good representation of what was proposed and merged. (Or perhaps a better algorithm is possible.) At any rate being timing-dependent, as the current system seems to be, is a bug.

[Martin Pool (mbp)]

Another case is <https://code.launchpad.net/~lifeless/launchpad/unbreak-bugs/+merge/67802>

[Robert Collins (lifeless)]

This may need a small bit of bzr using code, but thats pretty shallow and #bzr can help sort it out pretty quickly.