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.

• 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 become "hint" rows.
• Viewed the new web UI and re-parsed the change, saw "unreadable commit" messages.
• Viewed a good commit; reparsed a good commit.