Specify the object looked when calling .peel()
    
    In all of these calls we're doing object.peel(pygit2.Commit) to retrieve
    the first commit object related to the object itself.
    
    pygit2's documentation describes .peel() as:
      Object.peel(target_type) → Object
        Peel the current object and returns the first object of the given type
    
    Fixes https://pagure.io/pagure/issue/4325
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>