test: fix ForwardingPostReceiveHookTest relying on host git init.defaultBranch

Without setInitialBranch("main"), JGit uses its built-in default ("master"),
so refs/heads/main never gets created and RemoteRefUpdate fails in CI where
no global init.defaultBranch=main is set.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: coopernetes

jgit-proxy-core/src/test/java/org/finos/gitproxy/git/ForwardingPostReceiveHookTest.java

@@ -35,7 +35,10 @@ class ForwardingPostReceiveHookTest {
     @BeforeEach
     void setUp() throws Exception {
         // Local (proxy-side) repo
-        localGit = Git.init().setDirectory(localDir.toFile()).call();
+        localGit = Git.init()
+                .setDirectory(localDir.toFile())
+                .setInitialBranch("main")
+                .call();
         localRepo = localGit.getRepository();
         localRepo.getConfig().setBoolean("commit", null, "gpgsign", false);
         localRepo.getConfig().save();