Fix & polish app/sources/config.toml to include initial refspec config

Things like:
```toml
[frappe.passthru]
since = "version-14"
upstream = """
URL: https://github.com/frappe/frappe
Pull: +refs/heads/develop:refs/remotes/upstream/develop
Pull: +refs/heads/version-15:refs/remotes/upstream/version-15
Pull: +refs/heads/version-15-hotfix:refs/remotes/upstream/version-15-hotfix
Pull: +refs/tags/v15.*:refs/remotes/upstream/tags/v15.*
"""
```