Add cumulative diff API

Review Request #10233 — Created Oct. 15, 2018 and submitted — Latest diff uploaded

Information

Review Board
release-4.0.x
2edb57f...

Reviewers

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.