Merge proposal page hides reviewable changes by including merges in diffs

When new revisions are added after an initial proposal, a diff is made available, and that diff combines all revisions made between each comment. But some of those revisions may be merges from trunk, which means that they do not need review. The changes introduced by the merge are essentially noise, making it hard to see the changes that do need review.

A simple solution would be to combine only non-merge revisions together.