Previously we would show multiple FileDiffs for the same file in the

diff viewer if the file was modified in multiple commits. Now we

determine what FileDiffs are ancestors of other FileDiffs in the

commit history and filter out all the ancestors. The result is a

squashed diff between the original file and the resulting file.

Ran unit tests.
Manually verified that the squashed diffs were correct.
Still able to view the FileDiffs of review requests without

commit history.