Allow push-to-create repository functionality to create SHA256 repositories

Currently, pushing to create a project in GitLab fails in Gitaly with the following error:

fatal: the receiving end does not support this repository's hash algorithm
fatal: the remote end hung up unexpectedly

This is because CreateRepository gets called first to initialize an empty repository, which right now always gets initialized as sha1.

CreateRepository has a ObjectHash field that can be set to create a sha256 repo. So we need to modify the caller to call CreateRepository with the correct ObjectHash.

I'm unclear what is actually calling CreateRepository though. Is it workhorse/gitlab-shell or is it rails?

We would need to inspect the pack-protocol data coming from the client to see what object-format=<sha1|sha256> is, and set the ObjectHash field properly.