Refactoring of _request_branch method
    
    Nesting if-statements usually leads to worse maintainability and
    undestandability of a code. This commit refactors code in such
    a way that nested if-statements are removed.
    
    Merges: https://pagure.io/fedpkg/pull-request/475
    
    Signed-off-by: Dominik Rumian <drumian@redhat.com>