commit: reorganise commit hint strings

commit: reorganise commit hint strings

Strings of hint messages inserted into editor on interactive commit was
scattered in-line, rendering the code harder to understand at first
glance.

Extract those messages out into separate variables to make the code
outline easier to follow.

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