In the initial design of the FileAttachment in diffs feature, we had

four possible fields:

- added_in_filediff

- repository

- repo_path

- repo_revision
For newly-added files, we'd set the added_in_filediff relation. For

all others, we'd set the other three. There are two major problems with

this.
First, the "revision" for a modified file is unique when talking about

git (a blob SHA), but isn't in many other systems. For example, the

FileDiff.dest_detail for an SVN change might just be "(working copy)".

This would result in attachments corresponding to multiple revisions of

the diff stomping on each other.
Second, once we had the file, any FileAttachmentComments would not be

able to link back to the diff in question, because that information was

not available.
This change modifies things so we use the added_in_filediff relation

for both newly-added and the modified version of any binary files. We

still use the repository/path/revision fields for the original version

of files.

Ran unit tests.
Uploaded a diff with binary files and checked the relations.