We forget, compartmentalize or never bother to learn of the indignities and domestic terror of racism. Who remembers that in 1956 Nat King Cole, while performing for a white audience of 4,000 in Montgomery, Ala., was attacked by Klan members onstage?“The Green Book” remembers.The film’s title refers to “The Negro Motorist Green Book,” a travel guide for African-Americans sold in gas stations and …