Persist an old → new commit-hash mapping during migration

Persist an old → new commit-hash mapping during migration: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). When a repository is migrated to the new storage format, every commit is rewritten and gets a brand-new commit hash. Today there is no way to find out which…