For a long time, I've noticed that filenames are duplicated in the

review request emails. Having had my head in the commit code, I finally

realized why: we're iterating over DiffSet.files.all(), which includes

both the files in the commit(s) as well as the files in the cumulative

diff.
This change fixes it so we iterate over the cumulative_files property,

which shows only the FileDiffs in the cumulative diff.

Looked at the review request email previews and no longer saw duplicated

filenames.