Fix check unpushed changes in check_repo
    
    check_repo uses git-rev-list to check whether there are unpushed changes
    in current repo. Something goes wrong if user invokes fedpkg with --dist
    option. That is, revision range passed to git-rev-list will be
    origin/<dist>...<active_branch> rather than
    origin/<active_branch>...<active_branch>.
    
    Resolves: 1169663
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>