Download less of all-cabal-hashes

We can't just download the head of all-cabal-hashes, since a snapshot can request old revisions of Cabal files by hash (see #2175 and #2464).

But [as mentioned](https://github.com/commercialhaskell/stack/issues/2175#issuecomment-238704305), downloading the entire repo's history is a long-term problem.
Possible solution I proposed:

> I imagine one could add tags for each snapshot, and stack could fetch those tags when needed, but the tags would be needed remotely.