Fix displaying the branches available for a new PR in the requests template
    
    We have changed the format of the data returned by the endpoint listing
    the branches ready to be in a PR so that it also includes branches that
    are already in a PR (the info is then displayed on the front page of the
    project).
    However, this same internal API endpoint is also called in the page
    listing all the pull-requests and this page had not been adjusted for
    this data format change.
    
    This commit fixes that.
    
    Fixes https://pagure.io/pagure/issue/1310