Previously, FileDiffs for review requests with history would not show

cumulatively. That is, you would see only the result of applying that

FileDiff, not the result of applying the entire stack of FileDiffs

in the history graph.
We now compute the entire file history, including deletions, and use

that to compute the cumulative diff. In other words, for a review

request with commits A -> B -> C where all three commits modify the

same file, previously you would see the result of A, B, and C

independently. Now the diffviewer shows A, A..B, and A..C.
Files are still repeated (as indicated above) so this is not yet a true

cumulative diff. A future patch will update the diffviewer to only show

the leaf FileDiffs in the history graph.

Ran unit tests.
Uploaded a multi-commit review request and looked at the diff. I

  observed cumulative (but repeated) diffs.