Use list type for command in s:spawn() (#959)

This allows Neovim to bypass the shell and run git directly.
Vim still needs the shell because of how commands are collapsed on Windows and because setting the job's working directory via "cwd" does not work.

Refactored s:clone_opt to a list to make this possible.

plug.vim

@@ -1086,11 +1086,16 @@ function! s:update_impl(pull, force, args) abort
   normal! 2G
   silent! redraw
 
-  let s:clone_opt = get(g:, 'plug_shallow', 1) ?
-        \ '--depth 1' . (s:git_version_requirement(1, 7, 10) ? ' --no-single-branch' : '') : ''
+  let s:clone_opt = []
+  if get(g:, 'plug_shallow', 1)
+    call extend(s:clone_opt, ['--depth', '1'])
+    if s:git_version_requirement(1, 7, 10)
+      call add(s:clone_opt, '--no-single-branch')
+    endif
+  endif
 
   if has('win32unix') || has('wsl')
-    let s:clone_opt .= ' -c core.eol=lf -c core.autocrlf=input'
+    call extend(s:clone_opt, ['-c', 'core.eol=lf', '-c', 'core.autocrlf=input'])
   endif
 
   let s:submodule_opt = s:git_version_requirement(2, 8) ? ' --jobs='.threads : ''
@@ -1285,7 +1290,7 @@ function! s:spawn(name, cmd, opts)
     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]
+    let argv = a:cmd
     call extend(job, {
     \ 'on_stdout': function('s:nvim_cb'),
     \ 'on_stderr': function('s:nvim_cb'),
@@ -1301,7 +1306,10 @@ 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 cmd = join(map(copy(a:cmd), 'plug#shellescape(v:val, {"script": 0})'))
+    if has_key(a:opts, 'dir')
+      let cmd = s:with_cd(cmd, a:opts.dir, 0)
+    endif
     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]),
@@ -1415,21 +1423,29 @@ while 1 " Without TCO, Vim stack is bound to explode
     let [error, _] = s:git_validate(spec, 0)
     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', fetch_opt, prog), { 'dir': spec.dir })
+        let cmd = ['git', 'fetch']
+        if has_tag && !empty(globpath(spec.dir, '.git/shallow'))
+          call extend(cmd, ['--depth', '99999999'])
+        endif
+        if !empty(prog)
+          call add(cmd, prog)
+        endif
+        call s:spawn(name, cmd, { 'dir': spec.dir })
       else
         let s:jobs[name] = { 'running': 0, 'lines': ['Already installed'], 'error': 0 }
       endif
     else
       let s:jobs[name] = { 'running': 0, 'lines': s:lines(error), 'error': 1 }
     endif
   else
-    call s:spawn(name,
-          \ printf('git clone %s %s %s %s',
-          \ has_tag ? '' : s:clone_opt,
-          \ prog,
-          \ plug#shellescape(spec.uri, {'script': 0}),
-          \ plug#shellescape(s:trim(spec.dir), {'script': 0})), { 'new': 1 })
+    let cmd = ['git', 'clone']
+    if !has_tag
+      call extend(cmd, s:clone_opt)
+    endif
+    if !empty(prog)
+      call add(cmd, prog)
+    endif
+    call s:spawn(name, extend(cmd, [spec.uri, s:trim(spec.dir)]), { 'new': 1 })
   endif
 
   if !s:jobs[name].running
@@ -1466,7 +1482,7 @@ G_NVIM = vim.eval("has('nvim')") == '1'
 G_PULL = vim.eval('s:update.pull') == '1'
 G_RETRIES = int(vim.eval('get(g:, "plug_retries", 2)')) + 1
 G_TIMEOUT = int(vim.eval('get(g:, "plug_timeout", 60)'))
-G_CLONE_OPT = vim.eval('s:clone_opt')
+G_CLONE_OPT = ' '.join(vim.eval('s:clone_opt'))
 G_PROGRESS = vim.eval('s:progress_opt(1)')
 G_LOG_PROB = 1.0 / int(vim.eval('s:update.threads'))
 G_STOP = thr.Event()
@@ -2003,7 +2019,7 @@ function! s:update_ruby()
     end
   } if VIM::evaluate('s:mac_gui') == 1
 
-  clone_opt = VIM::evaluate('s:clone_opt')
+  clone_opt = VIM::evaluate('s:clone_opt').join(' ')
   progress = VIM::evaluate('s:progress_opt(1)')
   nthr.times do
     mtx.synchronize do