By Anirban Sen and Diane Bartz NEW YORK/WASHINGTON (Reuters) – Investment bankers and deal lawyers accustomed to regulatory hurdles to their mergers face an unprecedented challenge under U.S. President Joe Biden – antitrust watchdogs who are undaunted when they lose such battles in court. The U.S. Justice Department and Federal Trade Commission (FTC) have attempted to thwart 22 mergers since Biden came into office in January 2021, according to a Reuters review of announcements from the agencies. That outnumbers the antitrust challenges during the first two years of former President Barack Obam…