vim-patch:8.1.2115: MS-Windows: shell commands fail if &shell contains a space

Problem:    MS-Windows: shell commands fail if &shell contains a space.
Solution:   Use quotes instead of escaping. (closes vim/vim#4920)
2efc44b3f0

Always double-quote &shell if it contains a space.
Neovim does not support escaping space with backslash, unlike Vim.

N/A patches for version.c:

vim-patch:8.0.1455: if $SHELL contains a space then 'shell' is incorrect

Problem:    If $SHELL contains a space then the default value of 'shell' is
            incorrect. (Matthew Horan)
Solution:   Escape spaces in $SHELL. (Christian Brabandt, closes vim/vim#459)
4bfa8af141

vim-patch:8.2.1194: test failure because shell prompt differs

Problem:    Test failure because shell prompt differs.
Solution:   Set the shell prompt.
a4dc6f92bb

src/nvim/testdir/test_system.vim

@@ -1,5 +1,8 @@
 " Tests for system() and systemlist()
 
+source shared.vim
+source check.vim
+
 function! Test_System()
   if !executable('echo') || !executable('cat') || !executable('wc')
     return
@@ -88,3 +91,54 @@ function! Test_system_exmode()
   let a = system(v:progpath. cmd)
   call assert_notequal(0, v:shell_error)
 endfunc
+
+func Test_system_with_shell_quote()
+  throw 'skipped: enable after porting method patches'
+  CheckMSWindows
+
+  call mkdir('Xdir with spaces', 'p')
+  call system('copy "%COMSPEC%" "Xdir with spaces\cmd.exe"')
+
+  let shell_save = &shell
+  let shellxquote_save = &shellxquote
+  try
+    " Set 'shell' always needs noshellslash.
+    let shellslash_save = &shellslash
+    set noshellslash
+    let shell_tests = [
+          \ expand('$COMSPEC'),
+          \ '"' . fnamemodify('Xdir with spaces\cmd.exe', ':p') . '"',
+          \]
+    let &shellslash = shellslash_save
+
+    let sxq_tests = ['', '(', '"']
+
+    " Matrix tests: 'shell' * 'shellxquote'
+    for shell in shell_tests
+      let &shell = shell
+      for sxq in sxq_tests
+        let &shellxquote = sxq
+
+        let msg = printf('shell=%s shellxquote=%s', &shell, &shellxquote)
+
+        try
+          let out = 'echo 123'->system()
+        catch
+          call assert_report(printf('%s: %s', msg, v:exception))
+          continue
+        endtry
+
+        " On Windows we may get a trailing space and CR.
+        if out != "123 \n"
+          call assert_equal("123\n", out, msg)
+        endif
+
+      endfor
+    endfor
+
+  finally
+    let &shell = shell_save
+    let &shellxquote = shellxquote_save
+    call delete('Xdir with spaces', 'rf')
+  endtry
+endfunc