The width of the invisible PROMPT2 must take into account, in order
for user input to be aligned with the first line, that PROMPT1 can
contain newlines.

Author: Maxence Ahlouche
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/CAJeaomVyLSP_Wj%3D0FtYNTuoopWHyFarhUtYKDHs0HHv%2Bb%3DN9sA%40mail.gmail.com