Previously, when uploading a commit history to a review request, it was

impossible for your commit history to contain two commits which

modified the same file. This is because the FileDiff processing

checked the repository for the previous revisions of all files. In this

case, the repository would report that the file did not exist at the

given revision and validation would fail.
Now we can check for file existence within our current set of uploaded

DiffCommits. If we are unable to find an ancestor DiffCommit that

contains the file at the correct revision, we fall back to the old

behaviour of querying the repository. This allows linear commit

histories that modify the same file to be validated and uploaded.
NB: The diffviewer will not be able to show the FileDiff for such

DiffSets. This will be addressed in a later patch that will land

before this one.

Ran unit tests.
Posted a review request with commit history that modified the same file

in two different commits; it was successful.
Tried to view the review request's diff in the diff viewer; it was

unsuccesful.