Fix object of "rebased onto" comment
    
    When you rebase a pull request, you get an automatic comment
    that says "rebased onto <commit id>". However, the commit ID is
    always the first commit of the pull request, not the commit ID
    it was actually rebased onto. e.g. in
    https://pagure.io/fedora-qa/relval/pull-request/21
    right now the most recent message is "rebased onto 0fe88a2", and
    the first commit in the PR series is
    "Cleanup unreachble code for coverage reasons.", 0fe88a2. I think
    this should make it correct.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>