Summary: Depends on D19056. Fixes T8475. Ref T13054. Merges "ModernHunk" back into "Hunk".
Test Plan: Grepped for `modernhunk`. Reviewed revisions. Created a new revision. Used `bin/differential migrate-hunk` to migrate hunks between storage formats and back.
Maniphest Tasks: T13054, T8475
Differential Revision: https://secure.phabricator.com/D19057
