Jem Aswad Senior Music EditorIn his capacity as founder of the Revolt network, Sean “Diddy” Combs slammed General Motors for disrespecting Black-owned media brands in an open letter published Thursday, calling for more financial support in advertising and other spending.The move follows the placement of a full-page ad in Sunday’s Detroit Free Press, in which the leaders of Black-owned media companies — including Byron Allen and Ice Cube — accused GM CEO Mary Barra of racism for refusing to meet