When calculating the new parent_source_filename and

parent_source_revision fields in FileDiff.extra_data to record

parent file information, we were storing the data using the types

extracted from the diff. This meant that we could get a byte string,

instead of a Unicode string, and it also meant that we could get a

Revision instance. These weren't safe to put in extra_data (the

former issue triggering errors on Review Board 4.0 on Python 3, and the

latter triggering errors when uploading a file introduced in a parent

diff).
We now force these to be strings instead when we're ready to store them.

Unit tests pass.
Posted a commit (combining this change with another upcoming change) that

introduced a new file in a parent diff. It posted correctly, instead of

erroring out with a "not serializable" error for the Revision.