R1:8a4fbcd8c015

Provide a new "hint" table for weird commits (rewritten, unreadable)

Summary:
Ref T11522. This provides storage for tracking rewritten commits (new feature) and unreadable commits (existing feature, but really hacky).

This doesn't do anything yet, just adds a table and a CLI tool for updating it. I'll document the tool once it works. You just pipe in some JSON, but I need to document the format.

Test Plan:
- Piped JSON for "none", "rewritten" and "unreadable" hints into `bin/repository hint`.