Merge branch 'cw/no-detaching-an-unborn' into maint

"git checkout --detach", when you are still on an unborn branch,
should be forbidden, but it wasn't.

* cw/no-detaching-an-unborn:
  git-checkout: disallow --detach on unborn branch

Author: gitster

builtin/checkout.c

@@ -915,6 +915,8 @@ 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