t4150: am --resolved fails if index has no changes

Since 6d28644 (git-am: do not allow empty commits by mistake.,
2006-02-23), git-am --resolved will check to see if the index has any
changes to prevent the user from creating an empty commit by mistake.

Add a test for this.

Reviewed-by: Stefan Beller <sbeller@google.com>
Reviewed-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pyokagan

t/t4150-am.sh

@@ -425,6 +425,18 @@ test_expect_success 'am --resolved works' '
 	test_cmp expected another
 '
 
+test_expect_success 'am --resolved fails if index has no changes' '
+	rm -fr .git/rebase-apply &&
+	git reset --hard &&
+	git checkout lorem2^^ &&
+	test_must_fail git am lorem-move.patch &&
+	test_path_is_dir .git/rebase-apply &&
+	test_cmp_rev lorem2^^ HEAD &&
+	test_must_fail git am --resolved &&
+	test_path_is_dir .git/rebase-apply &&
+	test_cmp_rev lorem2^^ HEAD
+'
+
 test_expect_success 'am takes patches from a Pine mailbox' '
 	rm -fr .git/rebase-apply &&
 	git reset --hard &&