test/addSelect: add test for stage by select, line and marks

test/addSelect.vader

test/addSelect/addSelect_books_models_py_1_hunk_diff.expect

...addSelect/addSelect_books_models_py_2_vselect_diff.expect

test/addSelect/addSelect_books_models_py_3_lines_diff.expect

test/addSelect/addSelect_books_models_py_4_marks_diff.expect

test/test.config

test/utils.vim