Git: Branches

[eugenk]

Implement the following branching functionality

• List branches

  /:user/:repository/branches   GET index
  

• Show branch

  /:user/:repository/branches/:branch   GET show (identical with /:user/:repository/ref/:branch/commits)
  

• Create a branch

  /:user/:repository/branches   POST create
  

• Delete a branch

  /:user/:repository/branches/:branch   DELETE destroy
  

• Get the default branch

  /:user/:repository/branches///default   GET show
  

• Set the default branch

  /:user/:repository/branches///default PATCH update
  

These functions are already implemented in GraphQL by #165.

The struck out bullets are not needed because we only want to have read actions in the REST API.

[eugenk]

This is already done by #190.