Make `gitaly` the default branch for this project

Problem

The default branch name for Git projects is going to change. To what it's unclear, but master will be removed. From that moment on Git will have a different default branch. This creates trouble for user scripts depending on master to be the prime branch to develop against. Additionally GitLab, and with that Gitaly, needs to support default branches which aren't master. If there's issues there, we should be the first ones to find out and create a smooth transition for the rest of the Git community

Solution

One proposal is to use the repository name as default branch name. And conceptually "Merge my MR into gitaly" is something that makes sense. Gitaly's default branch should in my opinion be gitaly.

When repositories change their default branch, tooling depends on master still, and needs to be updated too. In Gitaly's case I'm thinking about the release tools, omnibus, CNG, GDK, etc.