Replace calls to .get_object() by calls to .peel()
    
    Apparently pygit2 dropped support for .get_object() in its release
    0.27.4.
    The .peel() method was introduced in 0.21.4 according to their changelog
    so it should be safe for use to just replace one by the other everywhere
    without needing some magic to use one or the other depending on the
    version.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>