Every year, roughly half of Indonesia’s 288 million people pack into trains, buses, ferries and cars and head home. The mudik, as the great Eid ul-Fitr exodus is known, is one of the largest annual human migrations on Earth and a ritual of return that defines the end of the holy fasting month of Ramadan. This year, the Ministry of Transportation estimates that 143.9 million journeys will be made. But it could hardly arrive at a worse time for the government’s finances. Oil prices have surged...