vim: fix shell options for cmd.exe

Reference:
junegunn/fzf#1018 (comment)

The command passed to cmd.exe is run as if the user typed it all
in an interactive prompt. This is convenient for testing and reduces the
burden to shellescape each token. This is enough for basic commands.
For non-trivial commands, use a batchfile.

autoload/dotvim8.vim

@@ -75,15 +75,10 @@ function! dotvim8#set_shell(shell)
 
   if shell ==# 'cmd.exe'
     let &shell = a:shell
-    set shellcmdflag=/c shellquote=
+    let &shellcmdflag = '/s /c'
+    let &shellxquote= '"'
+    set shellxescape= shellquote=
     let &shellredir = '>%s 2>&1'
-
-    if has('nvim')
-      set shellxquote= shellxescape=
-    else
-      set shellxquote=(
-      let &shellxescape = '"&|<>()@^'
-    endif
   elseif shell ==# 'powershell.exe'
     let &shell = a:shell
     let &shellcmdflag = '-NoProfile -NoLogo -Command'

shared.vim

@@ -257,6 +257,12 @@ if has('win32')
       return expand(s:base_dir . '/spell')
     endfunction
   endif
+
+  " Fix shell options for cmd.exe such that the command is run
+  " as if it was typed in an interactive prompt
+  if &shell =~# 'cmd.exe'
+    set shellcmdflag=/s\ /c shellxquote=\" shellxescape=
+  endif
 endif
 " }}}huge