Fix test.

dulwich/tests/test_porcelain.py

@@ -2834,4 +2834,4 @@ class MergeBaseTests(PorcelainTestCase):
         c1, c2, c3, c4 = build_commit_graph(
                 self.repo.object_store, [[1], [2, 1], [3, 1, 2], [4, 1]])
         self.assertEqual(
-            c1.id, porcelain.merge_base(self.repo, [c3.id, c4.id]))
+            [c1.id], porcelain.merge_base(self.repo, [c3.id, c4.id]))