avoid redirection warning when cloning anonymously
    
    Using `centpkg clone -a` results in a warning from git due to the URL
    being redirected:
    
        $ centpkg clone -a git
        Cloning into 'git'...
        warning: redirecting to https://gitlab.com/redhat/centos-stream/rpms/git.git/
        ...
    
    Update anongiturl to point to the real URL for HTTP cloning.  The
    addition of the trailing slash does not raise a warning, so it may be
    safely left off the URL.