am -3: support 3way merge on unborn branch

While on an unborn branch, git am -3 will fail to do a threeway merge as
it references HEAD as "our tree", but HEAD does not point to a valid
tree.

Fix this by using an empty tree as "our tree" when we are on an unborn
branch.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4151-am-abort.sh

@@ -83,4 +83,13 @@ test_expect_success 'am --abort will keep the local commits intact' '
 	test_cmp expect actual
 '
 
+test_expect_success 'am -3 stops on conflict on unborn branch' '
+	git checkout -f --orphan orphan &&
+	git reset &&
+	rm -f otherfile-4 &&
+	test_must_fail git am -3 0003-*.patch &&
+	test 2 -eq $(git ls-files -u | wc -l) &&
+	test 4 = "$(cat otherfile-4)"
+'
+
 test_done