what news broght the tall soldier to his comarades after going to the brook to wash his shirt

He heard that they were going to engage in combat with the enemy. Check out this quote from chapter one,

Once a certain tall soldier developed virtues and went resolutely to wash a shirt. He came flying back from a brook waving his garment bannerlike. He was swelled with a tale he had heard from a reliable friend, who had heard it from a truthful cavalryman, who had heard it from his trustworthy brother, one of the orderlies at division headquarters. He adopted the important air of a herald in red and gold.

"We're goin' t' move t'morrah--sure," he said pompously to a group in the company street. "We're goin' 'way up the river, cut across, an' come around in behint 'em."