commit https://github.com/vim/vim/commit/1000565c3a2439c9a7c9759284814dbf3b8bc20d Author: Bram Moolenaar <Bram@vim.org> Date: Thu Dec 31 21:03:23 2015 +0100 patch 7.4.1012 Problem: Vim overwrites the value of $PYTHONHOME. Solution: Do not set $PYTHONHOME if it is already set. (Kazuki Sakamoto, closes https://github.com/vim/vim/issues/500)

" Settings for test script execution
" Always use "COMMAND.COM", don't use the value of "$SHELL".
set shell=c:\COMMAND.COM shellquote= shellxquote= shellcmdflag=/c shellredir=>
" This is used only when the +eval feature is available.
if executable("cmd.exe")
   set shell=cmd.exe
endif