show the merge diff when proposing a merge

[Martin Pool]

When I'm proposing a merge and writing the cover letter, it would be really helpful if Launchpad showed me the diff of the merge I'm proposing. Then I can check that it's really what I intend to merge, and I can check that my cover letter has a good explanation of everything in the diff.

It would be nice if it also showed the to-be-merged revisions, though they could be done separately.

This would correspond to the workflow people get at present by looking at the diff generated by bzr send before submitting it.

For both this and actual reviews there's possibly an unresolved issue of having the diff and the text box in the same browser window, and therefore wanting to scroll through the diff while still having the text entry on screen...