KYIV, Ukraine — It was an early and delightful symbol of underdog resistance. Dubbed the “John Deere Brigade,” Ukrainian tractors were shown all over social media lugging away hastily abandoned Russian military equipment, from tanks to self-propelled artillery systems to complicated air defense platforms, worth tens of millions of dollars. Western predictions that Ukraine would fall to its invaders in as little as three days proved wildly off base. The breadbasket of Europe could punch above its weight. And now it was in the repo business.