This change makes two enhancements to the diff file attachment resource.
First, a new source_file argument allows uploading a file for the source

version of the FileDiff. In most cases we can allow the server to fetch

it from the repository, but in the case where there's a parent diff, the

uploading client needs to provide both the source and modified versions.
Second, I did run into a case where I successfully uploaded two files

for the same FileDiff, which caused errors later on. A new API error has

been added for this case.

Ran unit tests.
Posted binary file changes that used a parent diff and successfully

  uploaded both the source and modified versions.