[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.

poac/src/Cmd/Lint.cc

Lines 37 to 49 in f32c760

	// Read .gitignore if exists
	if (fs::exists(".gitignore")) {
	std::ifstream ifs(".gitignore");
	std::string line;
	while (std::getline(ifs, line)) {
	if (line.empty() || line[0] == '#') {
	continue;
	}
	cpplintCmd += " --exclude=";
	cpplintCmd += line;
	}
	}