commit: remove irrelavent prompt on `--allow-empty-message`

commit: remove irrelavent prompt on `--allow-empty-message`

Even when the `--allow-empty-message` option is given, "git commit"
offers an interactive editor session with prefilled message that says
the commit will be aborted if the buffer is emptied, which is wrong.

Remove the "an empty message aborts" part from the message when the
option is given to fix it.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Helped-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Hu Jialun <hujialun@comp.nus.edu.sg>
Signed-off-by: Junio C Hamano <gitster@pobox.com>