By Tassilo Hummel and Michel Rose PARIS (Reuters) -Momentum was building on Monday behind attempts by French left-wing parties to form a united front against President Emmanuel Macron in next month’s parliamentary elections, after radical Jean-Luc Melenchon clinched a deal with the Greens. Macron was re-elected for a second term last month, but he needs support from a majority of lawmakers to push through a pro-business and pro-EU legislative agenda that also includes a plan to increase the retirement age. Early polls for the June 12-19 elections suggest he could achieve that, but opposition p…