Add a method in pagure.lib.git to rebase pull-requests
    
    Basically, we clone the project_from (or remote git), add a new remote
    pointing to the targeted project, pull --rebase from the targeted project
    and branch and push --force to the original project_from branch_from.
    
    Nothing too strange in this
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>