IDifferencesFormSchema.selected_differences vocabulary is too restrictive

The vocabulary is generated at view initialization time from the
current batch. However, a difference selected on the page may, at form
submission time, no longer be in the batch.

To replicate the problem:

1. Go to +localpackagediffs.

2. Select a package that does not have any syncs outstanding (i.e. the
   checkbox on the left is enabled).

3. Expand it and change Ignored from "No" to "These versions".

4. In the filter form at the top, enter the name of tha package,
   select "Ignored packages", and submit. The package selected in (2)
   should be visible.

5. Expand it and change Ignored from "These versions" to "No".

6. Check the box on the left of the package name.

7. Hit the sync button at the bottom.

8. Observe that the page is submitted but that nothing actually
   happens.