Replace calls to pygit2.clone_repository by calls to git clone directly
    
    pygit2.clone_repository() is leaking file descriptors on large git repo
    making pagure generate "Too many open files" errors while it runs.
    This leads to certain task no longer working, especially the tasks
    around the tickets or PRs repositories of large project, lowering
    significantly their interest.
    With this commit we're now shelling out to git directly the cloning
    action, this should fix the leak of file descriptors and thus allow
    pagure to go back to its expected behaviour.
    
    Fixes https://pagure.io/pagure/issue/4277
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>