Support the new RawFileDiffData migration in condensediffs.

condensediffs has been updated to handle the migration of
LegacyFileDiffData entries to RawFileDiffData. A lot of work went into
optimizing this. These are converted in batches, with minimal SQL
queries on a modern database (read: not sqlite3).

A run of condensediffs on a copy of a real-world database containing
650,000 diffs (all stored in LegacyFileDiffData entries) took 15 minutes
to convert, with a space savings of 80% (3.6GB down to 714MB).

Reviewed at https://reviews.reviewboard.org/r/6227/