TestRepository: Support committing to unborn symref

Change-Id: I6bcee635adda7c929f6fa68ef20438fe77e49184

org.eclipse.jgit.junit/src/org/eclipse/jgit/junit/TestRepository.java

@@ -887,9 +887,8 @@ public class TestRepository<R extends Repository> {
 			branch = b;
 
 			Ref ref = db.getRef(branch.ref);
-			if (ref != null) {
+			if (ref != null && ref.getObjectId() != null)
 				parent(pool.parseCommit(ref.getObjectId()));
-			}
 		}
 
 		CommitBuilder(CommitBuilder prior) throws Exception {

org.eclipse.jgit.test/tst/org/eclipse/jgit/junit/TestRepositoryTest.java

@@ -265,6 +265,16 @@ public class TestRepositoryTest {
 		assertEquals("bar contents", blobAsString(amended, "bar"));
 	}
 
+	@Test
+	public void commitToUnbornHead() throws Exception {
+		repo.updateRef("HEAD").link("refs/heads/master");
+		RevCommit root = tr.branch("HEAD").commit().create();
+		Ref ref = repo.getRef(Constants.HEAD);
+		assertEquals(root, ref.getObjectId());
+		assertTrue(ref.isSymbolic());
+		assertEquals("refs/heads/master", ref.getTarget().getName());
+	}
+
 	private String blobAsString(AnyObjectId treeish, String path)
 			throws Exception {
 		RevObject obj = tr.get(rw.parseTree(treeish), path);