merge proposal form should show the unmerged revisions

[Martin Pool]

Suggestion from ~james_w during his rally talk: when proposing a merge, the cover letter and the proposed commit message are often just summaries of the revisions committed to the branch. In particular if you committed just one small change to the branch with a good message and you now want it merged, it is cruel to make the user retype that message.

We should instead
- show the to-be-merged revisions on that page
- perhaps prepopulate them into the commit message
- perhaps also show the diff while the user is proposing the review

Doing these well depends a bit on knowing where it's going to merge to, and might mean we need to shift the boundaries between the current workflow form. For instance perhaps the user should create the mp just by setting the target. Then they can see the diff and the pending revision, and then type the message. We would then only actually request review once the user is happy with the proposal. This could also eliminate some problems with people proposing the wrong target: they'll get a chance to see the diff is really big before it gets mailed to anyone else.