NEW ORLEANS — Louisiana’s governor on Wednesday posthumously pardoned Homer Plessy, the Black man whose arrest for refusing to leave a whites-only railroad car in 1892 led to the Supreme Court ruling that cemented “separate but equal” into U.S. law for half a century.The state Board of Pardons in November recommended the pardon for Plessy, who boarded the rail car as a member of a small civil rights group hoping to overturn a state law segregating trains. Instead, the protest led to the 1896 ruling …