When a review request is created with commit history, it is likely that

there will be FileDiffs in multiple commits that modify the same file.

For any FileDiff in a commit, all FileDiffs in previous commits are

known as "ancestor" FileDiffs.
These ancestor FileDiffs are now excluded from the diffviewer. This

removes the duplication of seeing the same file listed multiple times

with incremental changes. However, currently only the latest change for

that file is shown. True cumulative diffs are supported in /r/10130/.

Ran unit tests.
Viewed a review request with commit history and did not see repeated

  files.