Fix index-v1 repo commiting

Turns out that with index-v1.jar, it wasn't committing the repo metadata (name, description, icon, mirrors, etc). @cde spotted this while starting on #35. That is because I reused RepoUpdater.prepareRepoDetailsForSaving() but it has different logic. So I made that private, and implemented it in IndexV1Updater so it works.

This has minor security implications since it was not saving maxAge, which is used to check whether updates are being blocked.