Split diffs and context lines into groups based on None sentinel

def _split_diff(merge_result, context_lines=3):
    """Split diffs and context lines into groups based on None sentinel"""
    collect = []
    for item in _visible_in_diff(merge_result, context_lines=context_lines):
        if item is None:
            if collect:
                yield collect
            collect = []
        else:
            collect.append(item)