If a review request was uploaded with a FileDiff that contained a

parent diff that creates a 0 byte file, we would previously be unable to

generate the pre-patch file due to a limitation of patch(1). To work

around this, when we encounter a specific error from patch (that is,

when it only finds what it considers garbage in the input), we check to

see if the parent diff is empty and if so we return an empty buffer.

Future calls will get_original_file for that file will have no extra

database query overhead as the results of this (which costs at least two

queries) will be cached with the FileDiff.

Ran unit tests.
Uploaded a commit that contained a FileDiff that created a 0 byte

  file. With this patch applied, I was able to view the diff.