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.

Could this also fix and close #1016 (closed)?

How about a short unit test that loads a short fake index and ensures that the information ends up in the DB properly?

This does fix #1016 (closed) @grote

A test would definitely be nice, there is a RepoProviderTest right now but that doesn't handle this.

Could bundle a fake index in the repo to test.

Ok, added tests that cover the whole thing!

merged