When started as "vi", run in vi-compatibility mode, and don't source
defaults.vim.  This can be submitted upstream.

--- a/runtime/defaults.vim	2016-09-12 07:41:04.000000000 -0700
+++ b/runtime/defaults.vim	2016-09-15 15:55:10.820971765 -0700
@@ -13,6 +13,12 @@
   finish
 endif
 
+" When started as "vi", operate in compatible mode (unless you have a
+" .vimrc).
+if v:progname ==? "vi"
+  finish
+endif
+
 " Bail out if something that ran earlier, e.g. a system wide vimrc, does not
 " want Vim to use these default values.
 if exists('skip_defaults_vim')