Previously, we uploaded only per-commit FileDiffs to review requests

with history support. Now, we also require API clients to upload a

"cumulative diff" -- that is, a diff that represents the squashed commit

range -- to the draft diff API. This also results in the "finalization"

of the commit series, which prevents uploading of additional commits, as

this would result in a cumulative diff that does not represent the

entire uploaded range.

Ran unit tests.