pkgset: Correctly detect single tag for variant
    
    We need to check tags for the variant, not for the whole compose. This
    results in merge always being done even if there is a single tag. For
    f29 tag in Fedora this takes about 2 hours for each variant.
    
    Relates: https://pagure.io/pungi/issue/860
    Relates: https://bugzilla.redhat.com/show_bug.cgi?id=1551653
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>