[fmt] Ignore gitignored files by default and implement `--no-ignore-vcs` to disable it

[ken-matsui]

Like poac lint ignores gitignored files by default, poac fmt also wants to ignore those files. Also, we want --no-ignore-vcs to disable this default behavior.

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