patch 9.0.1440: "rvim" can execute a shell through :diffpatch

Problem:    "rvim" can execute a shell through :diffpatch.
Solution:   Disallow the shell "patch" command.
diff --git a/src/testdir/test_diffmode.vim b/src/testdir/test_diffmode.vim
index d60de50..cb042e1 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
Binary files differ