BERLIN—Chancellor Angela Merkel’s party on Saturday chose Armin Laschet, the governor of Germany’s most populous state, as its new leader—sending a signal of continuity months before an election in which voters will decide who becomes the new chancellor. Laschet defeated Friedrich Merz, a conservative and one-time Merkel rival, at an online convention of the Christian Democratic Union. Laschet won 521 votes to Merz’s 466. A third candidate, prominent lawmaker Norbert Roettgen, was eliminated in a first round of voting. Saturday’s vote isn’t the final word on who will run as the candidate for chancellor in Germany’s Sept. 26 election, but Laschet will either run for chancellor or will have a big say in who does. Merkel, who has been chancellor since 2005, announced in late 2018 that she wouldn’t seek a fifth term. She also stepped down from the CDU leadership. The decision ends an 11-month leadership limbo in Germany’s …