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.

• Piped JSON for "none", "rewritten" and "unreadable" hints into bin/repository hint.
• Examined the database to see that the table was written properly.
• Tried to pipe bad JSON in, invalid hint types, etc. Got reasonable human-readable error messages.