Sgt. Bowe Bergdahl, the former American prisoner of war in Afghanistan who was freed in a 2014 swap for five Taliban detainees, has asked President Obama to pardon him before leaving the White House to President-elect Donald J. Trump, who has called the soldier “a no-good traitor who should have been executed.” After the presidential election, Sergeant Bergdahl’s legal team submitted copies of a clemency application to the White House, the Justice Department and the Pentagon, according to White House and Justice Department officials. It requested a pre-emptive pardon that would avert Sergeant Bergdahl’s court-martial trial on charges of desertion and misbehavior. The trial is scheduled to begin on April 18.