June 4 (Reuters) - A Texas prosecutor asked the state's highest criminal court on Tuesday to reverse a pardon granted by Governor Gregg Abbott to a former U.S. Army sergeant convicted of murder and sentenced to prison for fatally shooting a Black Lives Matter protester. Travis County District Attorney Jose Garza, a Democrat whose office tried the murder case, said he asked the state Court of Criminal Appeals for a special order, called a writ of mandamus, to overturn Abbott's action. Garza told a news briefing in the state capital of Austin that Abbott, a Republican, violated the separation of...