The get_for_filediff method for getting file attachments attached to

diffs had a few major issues that were preventing it from functioning

correctly.
First, it was querying solely based on the repository, file path, and

revision. This meant we could theoretically end up returning

FileAttachment objects which were attached to a different review

request than the given FileDiff. While our database schema

theoretically would allow for a single FileAttachment to be attached to

multiple review requests, in practice everything assumes that it's more

of a foreign key relationship than a many to many relationship.
Second, when a review request is created with commit history, there are

actually multiple FileDiffs that all go into a single changed file. At

the very least, we'll have a FileDiff for the commit and a FileDiff for

the cumulative diff.
When we're posting changes from RBTools, we attach the FileAttachment to

the FileDiff corresponding to the commit. This meant that even though

the file was uploaded, it wasn't visible in the diff viewer because we'd

usually be looking at the cumulative diff.
This change makes it so FileAttachment.objects.get_for_filediff will

first try the simple case, and if that isn't found, it will check to see

if the DiffSet is created with commit history. If that's the case, we'll

look for the most recent FileDiff object corresponding to the given file

and use that instead.
Finally, we mostly were relying on this function to raise

ObjectDoesNotExist, but we also had a case where it could return

None. I've changed it to always return None when there are no

matching file attachments.

Made some queries to filediff API resources and saw that the serialized

  attachment fields no longer contained links to file attachments

  connected to other review requests.
Posted a change with RBTools that included binary files. Saw that I

  could now see those files in the diff viewer.
Ran unit tests.