diff::binary tests: empty diff when forced binary

Ensure that even when we're forcing a binary diff that we do not
assume that there *is* a diff.  There should be an empty diff for
no change.

tests/diff/binary.c

@@ -269,6 +269,36 @@ void test_diff_binary__delta_append(void)
 	git_index_free(index);
 }
 
+void test_diff_binary__empty_for_no_diff(void)
+{
+	git_diff_options opts = GIT_DIFF_OPTIONS_INIT;
+	git_oid id;
+	git_commit *commit;
+	git_tree *tree;
+	git_diff *diff;
+	git_buf actual = GIT_BUF_INIT;
+	const char *expected = "";
+
+	opts.flags = GIT_DIFF_SHOW_BINARY | GIT_DIFF_FORCE_BINARY;
+	opts.id_abbrev = GIT_OID_HEXSZ;
+
+	repo = cl_git_sandbox_init("renames");
+
+	cl_git_pass(git_oid_fromstr(&id, "19dd32dfb1520a64e5bbaae8dce6ef423dfa2f13"));
+	cl_git_pass(git_commit_lookup(&commit, repo, &id));
+	cl_git_pass(git_commit_tree(&tree, commit));
+
+	cl_git_pass(git_diff_tree_to_tree(&diff, repo, tree, tree, &opts));
+	cl_git_pass(git_diff_print(diff, GIT_DIFF_FORMAT_PATCH, git_diff_print_callback__to_buf, &actual));
+
+	cl_assert_equal_s("", actual.ptr);
+
+	git_buf_free(&actual);
+	git_diff_free(diff);
+	git_commit_free(commit);
+	git_tree_free(tree);
+}
+
 void test_diff_binary__index_to_workdir(void)
 {
 	git_index *index;