git_sort: Check if Patch-mainline tag exists
    
    If Patch-mainline and Git-commit tags are missing in the patch, sort script
    will fail with:
    
        IndexError: list index out of range
    
    This change ensures that Patch-mainline tag is present and if not, raise
    an error to warn the user.
    
    Signed-off-by: Alexandre Vicenzi <alexandre.vicenzi@suse.com>