[lint] Implement `--no-ignore-vcs` to disable automatic gitignore read

[ken-matsui]

poac lint reads .gitignore by default if it exists, but we may sometimes want to avoid this behavior.

https://github.com/poac-dev/poac/blob/f32c76033c9d70f7b36440e36590c30fda5930ce/src/Cmd/Lint.cc#L37-L49