git-checkout: disallow --detach on unborn branch

abe199808c (git checkout -b: allow switching out of an unborn branch)
introduced a bug demonstrated by

  git checkout --orphan foo
  git checkout --detach
  git symbolic-ref HEAD

which gives 'refs/heads/(null)'.

This happens because we strbuf_addf(&branch_ref, "refs/heads/%s",
opts->new_branch) when opts->new_branch can be NULL for --detach.

Catch and forbid this case, adding a test to t2017 to catch it in
future.

Signed-off-by: Chris Webb <chris@arachsys.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -921,6 +921,8 @@ static int switch_unborn_to_new_branch(struct checkout_opts *opts)
 	int status;
 	struct strbuf branch_ref = STRBUF_INIT;
 
+	if (!opts->new_branch)
+		die(_("You are on a branch yet to be born"));
 	strbuf_addf(&branch_ref, "refs/heads/%s", opts->new_branch);
 	status = create_symref("HEAD", branch_ref.buf, "checkout -b");
 	strbuf_release(&branch_ref);

t/t2017-checkout-orphan.sh

@@ -116,4 +116,10 @@ test_expect_success '--orphan refuses to switch if a merge is needed' '
 	git reset --hard
 '
 
+test_expect_success 'cannot --detach on an unborn branch' '
+	git checkout master &&
+	git checkout --orphan new &&
+	test_must_fail git checkout --detach
+'
+
 test_done