Make the first branch pushed to an empty repository the default HEAD

In an empty repository, pushing a new branch not called "master" would leave HEAD in an unknown state, causing ambiguity if another branch were pushed. This could in turn cause a new protected branch to be created and cause the default branch to change.

* Closes #1561
* Closes #1576
* Closes https://github.com/gitlabhq/gitlabhq/issues/8883

See merge request !614