Run `./t/interact.pl` and it will build a database from `t/tree` and
leave you in a shell so you can experiment. When you exit the shell,
the temporary repo and database will be cleaned up.
Also:
- TestEnvironment: add report() function to produce a human-readable
summary of the environment
- TestHelpers: turn on `strict` and `warnings` so the caller doesn't
have to manually `use strict` and `use warnings`.
