In our code for getting the file attachments to display in the diff

viewer, we assume that when an interdiff is forced, an interfilediff

will be present. But this isn't the case for reverted file diffs. We

had an assertion for this that failed for the case of reverted files.
This change fixes that assumption and grabs the right file attachments

for reverted file diffs, grabbing the modified one as the original and

the original as the modified.

Tested viewing an interdiff with reverted binary files, saw that the

  diffs worked and the assertion no longer failed.