Support list type command for s:system to reduce batchfiles on Windows (#956)

* 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.
Neovim's system() does not support working directory system()
so consider refactoring s:system to use a synchronous job.

* Do not escape simple shell arguments

Regexp taken from vim-fugitive s:shellesc().

* Set shellredir on Windows

Prep to use list type for command  passed to s:system() within s:spawn()

* Internalize shellredir for s:spawn

s:spawn needs to redirect stderr to stdout for jobs callbacks
but s:system (for old Vim versions) sets shellredir if needed.

* Leverage job api for cwd and stderr

Vim/Neovim support stderr redirection and support error callbacks.
Vim 8 and Neovim can set a job's working directory via 'cwd' key
but it cannot be used as is on Vim because CI fails for the Vim release in Ubuntu Bionic and the latest Vim release.

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
@@ -864,8 +864,15 @@ endfunction
 
 function! s:chsh(swap)
   let prev = [&shell, &shellcmdflag, &shellredir]
-  if !s:is_win && a:swap
-    set shell=sh shellredir=>%s\ 2>&1
+  if !s:is_win
+    set shell=sh
+  endif
+  if a:swap
+    if &shell =~# 'powershell\.exe' || &shell =~# 'pwsh$'
+      let &shellredir = '2>&1 | Out-File -Encoding UTF8 %s'
+    elseif &shell =~# 'sh' || &shell =~# 'cmd\.exe'
+      set shellredir=>%s\ 2>&1
+    endif
   endif
   return prev
 endfunction
@@ -898,7 +905,7 @@ 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 +968,7 @@ 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)
@@ -1264,7 +1271,7 @@ function! s:job_cb(fn, job, ch, data)
 endfunction
 
 function! s:nvim_cb(job_id, data, event) dict abort
-  return a:event == 'stdout' ?
+  return (a:event == 'stdout' || a:event == 'stderr') ?
     \ s:job_cb('s:job_out_cb',  self, 0, join(a:data, "\n")) :
     \ s:job_cb('s:job_exit_cb', self, 0, a:data)
 endfunction
@@ -1273,12 +1280,15 @@ function! s:spawn(name, cmd, opts)
   let job = { 'name': a:name, 'running': 1, 'error': 0, 'lines': [''],
             \ 'new': get(a:opts, 'new', 0) }
   let s:jobs[a:name] = job
-  let cmd = has_key(a:opts, 'dir') ? s:with_cd(a:cmd, a:opts.dir, 0) : a:cmd
-  let argv = s:is_win ? ['cmd', '/s', '/c', '"'.cmd.'"'] : ['sh', '-c', cmd]
 
   if s:nvim
+    if has_key(a:opts, 'dir')
+      let job.cwd = a:opts.dir
+    endif
+    let argv = s:is_win ? ['cmd', '/s', '/c', '"'.a:cmd.'"'] : ['sh', '-c', a:cmd]
     call extend(job, {
     \ 'on_stdout': function('s:nvim_cb'),
+    \ 'on_stderr': function('s:nvim_cb'),
     \ 'on_exit':   function('s:nvim_cb'),
     \ })
     let jid = s:plug_call('jobstart', argv, job)
@@ -1291,9 +1301,13 @@ function! s:spawn(name, cmd, opts)
             \ 'Invalid arguments (or job table is full)']
     endif
   elseif s:vim8
+    let cmd = has_key(a:opts, 'dir') ? s:with_cd(a:cmd, a:opts.dir, 0) : a:cmd
+    let argv = s:is_win ? ['cmd', '/s', '/c', '"'.cmd.'"'] : ['sh', '-c', cmd]
     let jid = job_start(s:is_win ? join(argv, ' ') : argv, {
     \ 'out_cb':   function('s:job_cb', ['s:job_out_cb',  job]),
+    \ 'err_cb':   function('s:job_cb', ['s:job_out_cb',  job]),
     \ 'exit_cb':  function('s:job_cb', ['s:job_exit_cb', job]),
+    \ 'err_mode': 'raw',
     \ 'out_mode': 'raw'
     \})
     if job_status(jid) == 'run'
@@ -1304,7 +1318,7 @@ function! s:spawn(name, cmd, opts)
       let job.lines   = ['Failed to start job']
     endif
   else
-    let job.lines = s:lines(call('s:system', [cmd]))
+    let job.lines = s:lines(call('s:system', has_key(a:opts, 'dir') ? [a:cmd, a:opts.dir] : [a:cmd]))
     let job.error = v:shell_error != 0
     let job.running = 0
   endif
@@ -1402,7 +1416,7 @@ while 1 " Without TCO, Vim stack is bound to explode
     if empty(error)
       if pull
         let fetch_opt = (has_tag && !empty(globpath(spec.dir, '.git/shallow'))) ? '--depth 99999999' : ''
-        call s:spawn(name, printf('git fetch %s %s 2>&1', fetch_opt, prog), { 'dir': spec.dir })
+        call s:spawn(name, printf('git fetch %s %s', fetch_opt, prog), { 'dir': spec.dir })
       else
         let s:jobs[name] = { 'running': 0, 'lines': ['Already installed'], 'error': 0 }
       endif
@@ -1411,7 +1425,7 @@ while 1 " Without TCO, Vim stack is bound to explode
     endif
   else
     call s:spawn(name,
-          \ printf('git clone %s %s %s %s 2>&1',
+          \ printf('git clone %s %s %s %s',
           \ has_tag ? '' : s:clone_opt,
           \ prog,
           \ plug#shellescape(spec.uri, {'script': 0}),
@@ -2055,7 +2069,23 @@ 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, ...)
+  if a:arg =~# '^[A-Za-z0-9_/:.-]\+$'
+    return a:arg
+  endif
   let opts = a:0 > 0 && type(a:1) == s:TYPE.dict ? a:1 : {}
   let shell = get(opts, 'shell', s:is_win ? 'cmd.exe' : 'sh')
   let script = get(opts, 'script', 1)
@@ -2105,8 +2135,24 @@ 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 s:is_win
+    if type(a:cmd) == s:TYPE.list
+      " 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 +2205,10 @@ function! s:git_validate(spec, check_branch)
               \ branch, a:spec.branch)
       endif
       if empty(err)
-        let [ahead, behind] = split(s:lastline(s:system(printf(
-              \ 'git rev-list --count --left-right HEAD...origin/%s',
-              \ a:spec.branch), a:spec.dir)), '\t')
+        let [ahead, behind] = split(s:lastline(s:system([
+        \ 'git', 'rev-list', '--count', '--left-right',
+        \ 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 +2232,9 @@ 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 +2343,7 @@ 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 +2538,13 @@ 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 '
-      \ . (s:git_version_requirement(2, 10, 0) ? '--no-show-signature ' : '')
-      \ . join(map(['--pretty=format:%x01%h%x01%d%x01%s%x01%cr', range], 'plug#shellescape(v:val)'))
+      let cmd = ['git', 'log', '--graph', '--color=never']
+      if s:git_version_requirement(2, 10, 0)
+        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 +2612,7 @@ 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