Add .gitignore to .verify-helper/.gitignore
If you don't add this, `.gitignore` confuses users if they need to
commit the `.gitignore`.
@@ -167,6 +167,7 @@ def subcommand_docs() -> None:
def generate_gitignore() -> None:
path = pathlib.Path('.verify-helper/.gitignore')
data = textwrap.dedent("""\
+ .gitignore
cache/
include/
markdown/
