A remastered version of Ken Burns’s masterful documentary, “The Civil War,” began airing on PBS yesterday to mark the 25th anniversary of the film and the 150th anniversary of the end of the conflict. I wrote about the process of remastering the film, and Burns’s opportunity to make the movie he always dreamed of, on Friday. All week, I’ll be running excerpts of a long conversation we had about “The Civil War,” race, violence, photography and historical scholarship. This is the first installment.