Add a bin/hunks script to manage migrations of hunk data
Ref T4045. Ref T5179. While we'll eventually need to force a migration, we can let installs (particularly large installs) do an online migration for now. This moves hunks to the new storage format one at a time.
(Note that nothing writes to the new store yet, so this is the only way to populate it.)
Test Plan: Added a break; after migrating one row and moved a few rows over. Spot checked them in the database and viewed the affected diffs.
Reviewed By: btrahan
Differential Revision: https://secure.phabricator.com/D9291