Add API endpoint to set the default git branch and expose it in an existing endpoint
    
    This commit adds a "default" field to api/0/<namespace>/<name>/git/branches
    which returns the name of the default branch and potentially its HEAD
    commit hash.
    
    It also adds a new API endpoint allowing to change this default branch
    to any other existing branch.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>