November saw Pfizer Inc. announce its plans to merge with Allergan PLC in a $155 billion deal that would create the world's biggest drug maker and mark history's second-largest deal ever. A steady stream of activity since then has left global merger-and-acquisition volume within a whisker of the 2007 record of $4.6 trillion, while U.S. calendar-year M&A topped the $2 trillion level for the first time ever in November, according to Dealogic.