s:system supports list type for command

Objective is to reduce batchfiles on Windows.
List type gives more flexibility on s:system()
on how to pass the shell command to the builtin system().
If system() supports list type for command
and there is no working directory, run it directly on system().
Targets Neovim only.
Else, convert the list to an escaped command
so that the user's shell can execute it.

plug.vim

@@ -372,7 +372,7 @@ endfunction
 
 function! s:git_version_requirement(...)
   if !exists('s:git_version')
-    let s:git_version = map(split(split(s:system('git --version'))[2], '\.'), 'str2nr(v:val)')
+    let s:git_version = map(split(split(s:system(['git', '--version']))[2], '\.'), 'str2nr(v:val)')
   endif
   return s:version_requirement(s:git_version, a:000)
 endfunction
@@ -898,7 +898,7 @@ function! s:regress_bar()
 endfunction
 
 function! s:is_updated(dir)
-  return !empty(s:system_chomp('git log --pretty=format:"%h" "HEAD...HEAD@{1}"', a:dir))
+  return !empty(s:system_chomp(['git', 'log', '--pretty=format:%h', 'HEAD...HEAD@{1}'], a:dir))
 endfunction
 
 function! s:do(pull, force, todo)
@@ -961,7 +961,7 @@ endfunction
 
 function! s:checkout(spec)
   let sha = a:spec.commit
-  let output = s:system('git rev-parse HEAD', a:spec.dir)
+  let output = s:system(['git', 'rev-parse', 'HEAD'], a:spec.dir)
   if !v:shell_error && !s:hash_match(sha, s:lines(output)[0])
     let output = s:system(
           \ 'git fetch --depth 999999 && git checkout '.plug#shellescape(sha).' --', a:spec.dir)
@@ -2055,6 +2055,19 @@ function! s:shellesc_sh(arg)
   return "'".substitute(a:arg, "'", "'\\\\''", 'g')."'"
 endfunction
 
+" Escape the shell argument based on the shell.
+" Vim and Neovim's shellescape() are insufficient.
+" 1. shellslash determines whether to use single/double quotes.
+"    Double-quote escaping is fragile for cmd.exe.
+" 2. It does not work for powershell.
+" 3. It does not work for *sh shells if the command is executed
+"    via cmd.exe (ie. cmd.exe /c sh -c command command_args)
+" 4. It does not support batchfile syntax.
+"
+" Accepts an optional dictionary with the following keys:
+" - shell: same as Vim/Neovim 'shell' option.
+"          If unset, fallback to 'cmd.exe' on Windows or 'sh'.
+" - script: If truthy and shell is cmd.exe, escape for batchfile syntax.
 function! plug#shellescape(arg, ...)
   let opts = a:0 > 0 && type(a:1) == s:TYPE.dict ? a:1 : {}
   let shell = get(opts, 'shell', s:is_win ? 'cmd.exe' : 'sh')
@@ -2105,8 +2118,24 @@ function! s:system(cmd, ...)
   let batchfile = ''
   try
     let [sh, shellcmdflag, shrd] = s:chsh(1)
-    let cmd = a:0 > 0 ? s:with_cd(a:cmd, a:1) : a:cmd
+    if type(a:cmd) == s:TYPE.list
-    if s:is_win
+      " Neovim's system() supports list argument to bypass the shell
+      " but it cannot set the working directory for the command.
+      " Assume that the command does not rely on the shell.
+      if has('nvim') && a:0 == 0
+        return system(a:cmd)
+      endif
+      let cmd = join(map(copy(a:cmd), 'plug#shellescape(v:val, {"shell": &shell, "script": 0})'))
+      if &shell =~# 'powershell\.exe'
+        let cmd = '& ' . cmd
+      endif
+    else
+      let cmd = a:cmd
+    endif
+    if a:0 > 0
+      let cmd = s:with_cd(cmd, a:1, type(a:cmd) != s:TYPE.list)
+    endif
+    if s:is_win && type(a:cmd) != s:TYPE.list
       let [batchfile, cmd] = s:batchfile(cmd)
     endif
     return system(cmd)
@@ -2159,9 +2188,10 @@ function! s:git_validate(spec, check_branch)
               \ branch, a:spec.branch)
       endif
       if empty(err)
-        let [ahead, behind] = split(s:lastline(s:system(printf(
+        let [ahead, behind] = split(s:lastline(s:system([
-              \ 'git rev-list --count --left-right HEAD...origin/%s',
+        \ 'git', 'rev-list', '--count', '--left-right',
-              \ a:spec.branch), a:spec.dir)), '\t')
+        \ printf('HEAD...origin/%s', a:spec.branch)
+        \ ], a:spec.dir)), '\t')
         if !v:shell_error && ahead
           if behind
             " Only mention PlugClean if diverged, otherwise it's likely to be
@@ -2185,7 +2215,9 @@ endfunction
 
 function! s:rm_rf(dir)
   if isdirectory(a:dir)
-    call s:system((s:is_win ? 'rmdir /S /Q ' : 'rm -rf ') . plug#shellescape(a:dir))
+    call s:system(s:is_win
+    \ ? 'rmdir /S /Q '.plug#shellescape(a:dir)
+    \ : ['rm', '-rf', a:dir])
   endif
 endfunction
 
@@ -2294,7 +2326,7 @@ function! s:upgrade()
   let new = tmp . '/plug.vim'
 
   try
-    let out = s:system(printf('git clone --depth 1 %s %s', plug#shellescape(s:plug_src), plug#shellescape(tmp)))
+    let out = s:system(['git', 'clone', '--depth', '1', s:plug_src, tmp])
     if v:shell_error
       return s:err('Error upgrading vim-plug: '. out)
     endif
@@ -2489,11 +2521,13 @@ function! s:diff()
     call s:append_ul(2, origin ? 'Pending updates:' : 'Last update:')
     for [k, v] in plugs
       let range = origin ? '..origin/'.v.branch : 'HEAD@{1}..'
-      let cmd = 'git log --graph --color=never '
+      let cmd = ['git', 'log', '--graph', '--color=never']
-      \ . (s:git_version_requirement(2, 10, 0) ? '--no-show-signature ' : '')
+      if s:git_version_requirement(2, 10, 0)
-      \ . join(map(['--pretty=format:%x01%h%x01%d%x01%s%x01%cr', range], 'plug#shellescape(v:val)'))
+        call add(cmd, '--no-show-signature')
+      endif
+      call extend(cmd, ['--pretty=format:%x01%h%x01%d%x01%s%x01%cr', range])
       if has_key(v, 'rtp')
-        let cmd .= ' -- '.plug#shellescape(v.rtp)
+        call extend(cmd, ['--', v.rtp])
       endif
       let diff = s:system_chomp(cmd, v.dir)
       if !empty(diff)
@@ -2561,7 +2595,7 @@ function! s:snapshot(force, ...) abort
   let names = sort(keys(filter(copy(g:plugs),
         \'has_key(v:val, "uri") && !has_key(v:val, "commit") && isdirectory(v:val.dir)')))
   for name in reverse(names)
-    let sha = s:system_chomp('git rev-parse --short HEAD', g:plugs[name].dir)
+    let sha = s:system_chomp(['git', 'rev-parse', '--short', 'HEAD'], g:plugs[name].dir)
     if !empty(sha)
       call append(anchor, printf("silent! let g:plugs['%s'].commit = '%s'", name, sha))
       redraw