Why would a blockchain be necessary, (as opposed to a simpler distributed storage mechanism for git repos)?

If you want to have a main canonical state of the "maintained" repo, that is mutable, similar to what we currently have with Github, a blockchain would be one way to achieve that.

Usually you can sign your work in a cryptographically verifiable way https://git-scm.com/book/en/v2/Git-Tools-Signing-Your-Work

Then you are still missing a way to discover what the "one canonical state" of a repository is. There could be hundreds of different signed branches and versions of the repository the maintainer worked on at one point, with most of them being abandoned work.

Let's ignore for a moment the utility of Github issues, etc. and focus on why Github repos are convenient just for the code.

- You find the repo either via link or some other discovery mechanism like Google

- You do a small sanity check to see if the repo is the one you actually wanted

- You clone the repo and start using it from the default branch that was set by the maintainer -> DONE

Achieving a similar flow is not possible if you just fling out signed commits into the world. Sure, there are also different ways on how you can publish the "one canonical state", but as soon as multiple maintainers and contributors get involved, a blockchain will look more and more like a fitting solution.

Not sure I understand - doesn't git provide all the functionality you need anyway?

Better: https://github.com/ipfs-shipyard/igis && https://github.com/magik6k/git-remote-ipld

How long does it take to clone linux.git from there?

Ha!

http://oscoin.io/ a team in Berlin is working on it for some time.

Git?