Outwardly, Donald J. Trump called it a “unity meeting” — a closed-door session in Washington on Thursday involving his own inner circle and the Republican National Committee’s high command. Inside, however, it was more of a clearing of the air, according to three people briefed in detail on the discussion. And the candid remarks included some by Mr. Trump directed at his own team. There was plenty of tension to defuse: For months, Mr. Trump has denounced the party’s major donors, and only this week he went back on a written pledge to support whoever becomes the Republican presidential nominee...