User the user's default email when rebasing
    
    We used to use a dummy email address when rebasing. With this
    commit we are instead using the default email address of the user
    asking for the rebase.
    
    Fixes https://pagure.io/pagure/issue/4379
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>