tests-pr.yml: Fetch upstream master and compare against it
    
    Rather than relying on origin/master, fetch a fresh master branch from
    the upstream repo by URL. After the Fedora CI migration to testing farm,
    origin == the fork and not the target repository. This should work
    around it.
    
    See also:
    https://pagure.io/fedora-ci/general/issue/158
    
    Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>