Only consider the most recently active branches for new PR
    
    Instead of considering all the branches in the git repository when we
    are looking for the branches available to open new PRs from, we are
    now considering only the 8 latest active branches. This should
    optimize the "New PR" drop-down button in the UI quite significantly in
    large repositories.
    
    Fixes https://pagure.io/pagure/issue/4587
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>