Do not always compute the list of diff commits
    
    If the repo has a branch but no main branch (defaults to master), then
    we end up going through all the commits of the branch and say that they
    aren't in the main branch, which make sense since there are no main
    branch. But this leads to an error when trying to build the URL to open
    the pull-request between that main branch and the current one.
    
    So if there are no main branch, don't compute the list of different
    commits and don't show the notification about potential PR, just show
    the list of commits as if it was all good.
    
    Fixes https://pagure.io/pagure/issue/1441