git

- `raise "string"` raises a `RuntimeError` - no need to be explicit
- Remove top-level comment in the `RevList` class
- Use `%w()` instead of `%w[]`
- Extract an `environment_variables` method to cache `env.slice(*ALLOWED_VARIABLES)`
- Use `start_with?` for env variable validation instead of regex match
- Validation specs for each allowed environment variable were identical. Build them dynamically.
- Minor change to `popen3` expectation.