R1:e4c4724afd6b

Migrate the "badcommit" table to use the less-hacky "hint" mechanism

Summary: Ref T11522. This migrates any "badcommit" data (which probably only exists at Facebook and on 1-2 other installs in the wild) to the new "hint" table.

Test Plan:
- Wrote some bad commit annotations to the badcommit table.
- Viewed them in the web UI and used `bin/repository reparse --change ...` to reparse them. Saw "this is bad" messages.
- Ran migration, verified that valid "badcommit" rows were successfully migrated to…