vcs export --exact-with-tags output fails pre-commit linting rules

[jwdinius]

When I run vcs export --exact-with-tags src > repos.pegged the resulting file repos.pegged correctly replaces the version field for all of my repositories with the commit hash (or tag descriptor, if applicable) of my source dependencies.

However, my repository uses pre-commit hooks for change validation and the resulting file is missing the leading --- that the yaml linter expects.

I think that the fix is simple here and I will put up a PR.