Use job API for system() calls in GUI Vim to avoid overhead

In GUI Vim (gvim, MacVim), each system() call incurs significant
overhead due to command prompt window creation, making operations
like PlugDiff ~5x slower than in terminal Vim or Neovim.

Add s:system_job() which uses job_start with file-based output to
run commands without that overhead, and use it in s:system() when
running in GUI Vim with job support.

Since v:shell_error is read-only in Vim 9, introduce s:shell_error
which is set by both code paths and used at all call sites.

Fixes #1312

plug.vim

@@ -68,6 +68,7 @@ let s:mac_gui = has('gui_macvim') && has('gui_running')
 let s:is_win = has('win32')
 let s:nvim = has('nvim-0.2') || (has('nvim') && exists('*jobwait') && !s:is_win)
 let s:vim8 = has('patch-8.0.0039') && exists('*job_start')
+let s:shell_error = 0
 if s:is_win && &shellslash
   set noshellslash
   let s:me = resolve(expand('<sfile>:p'))
@@ -170,7 +171,7 @@ function! s:git_origin_branch(spec)
 
   " The command may not return the name of a branch in detached HEAD state
   let result = s:lines(s:system('git symbolic-ref --short HEAD', a:spec.dir))
-  return v:shell_error ? '' : result[-1]
+  return s:shell_error ? '' : result[-1]
 endfunction
 
 if s:is_win
@@ -1097,7 +1098,7 @@ function! s:checkout(spec)
     let credential_helper = s:disable_credential_helper() ? '-c credential.helper= ' : ''
     let output = s:system(
           \ 'git '.credential_helper.'fetch --depth 999999 && git checkout '.plug#shellescape(sha).' --', a:spec.dir)
-    let error = v:shell_error
+    let error = s:shell_error
   endif
   return [output, error]
 endfunction
@@ -1303,7 +1304,7 @@ function! s:update_finish()
         let tag = spec.tag
         if tag =~ '\*'
           let tags = s:lines(s:system('git tag --list '.plug#shellescape(tag).' --sort -version:refname 2>&1', spec.dir))
-          if !v:shell_error && !empty(tags)
+          if !s:shell_error && !empty(tags)
             let tag = tags[0]
             call s:log4(name, printf('Latest tag for %s -> %s', spec.tag, tag))
             call append(3, '')
@@ -1311,7 +1312,7 @@ function! s:update_finish()
         endif
         call s:log4(name, 'Checking out '.tag)
         let out = s:system('git checkout -q '.plug#shellescape(tag).' -- 2>&1', spec.dir)
-        let error = v:shell_error
+        let error = s:shell_error
       endif
       if !error && filereadable(spec.dir.'/.gitmodules') &&
             \ (s:update.force || has_key(s:update.new, name) || s:is_updated(spec.dir))
@@ -1319,7 +1320,7 @@ function! s:update_finish()
         let out .= s:bang('git submodule update --init --recursive'.s:submodule_opt.' 2>&1', spec.dir)
         let error = v:shell_error
       endif
-      let msg = s:format_message(v:shell_error ? 'x': '-', name, out)
+      let msg = s:format_message(error ? 'x': '-', name, out)
       if error
         call add(s:update.errors, name)
         call s:regress_bar()
@@ -1480,7 +1481,7 @@ function! s:spawn(name, spec, queue, opts)
     endif
   else
     let job.lines = s:lines(call('s:system', has_key(a:opts, 'dir') ? [argv, a:opts.dir] : [argv]))
-    let job.error = v:shell_error != 0
+    let job.error = s:shell_error != 0
     let job.running = 0
   endif
 endfunction
@@ -2334,6 +2335,21 @@ function! s:with_cd(cmd, dir, ...)
   return printf('%s %s %s %s', cd, plug#shellescape(a:dir, {'script': script, 'shell': &shell}), sep, a:cmd)
 endfunction
 
+function! s:system_job(cmd) abort
+  let tmp = tempname()
+  let job = job_start(['/bin/sh', '-c', a:cmd], {
+  \ 'out_io': 'file',
+  \ 'out_name': tmp,
+  \ 'err_io': 'out',
+  \})
+  while job_status(job) ==# 'run'
+  endwhile
+  let s:shell_error = job_info(job).exitval
+  let result = filereadable(tmp) ? join(readfile(tmp, 'b'), "\n") : ''
+  silent! call delete(tmp)
+  return result
+endfunction
+
 function! s:system(cmd, ...)
   let batchfile = ''
   try
@@ -2343,7 +2359,9 @@ function! s:system(cmd, ...)
       " 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)
+        let ret = system(a:cmd)
+        let s:shell_error = v:shell_error
+        return ret
       endif
       let cmd = join(map(copy(a:cmd), 'plug#shellescape(v:val, {"shell": &shell, "script": 0})'))
       if s:is_powershell(&shell)
@@ -2358,7 +2376,12 @@ function! s:system(cmd, ...)
     if s:is_win && type(a:cmd) != s:TYPE.list
       let [batchfile, cmd] = s:batchfile(cmd)
     endif
-    return system(cmd)
+    if s:vim8 && has('gui_running') && !s:is_win
+      return s:system_job(cmd)
+    endif
+    let ret = system(cmd)
+    let s:shell_error = v:shell_error
+    return ret
   finally
     let [&shell, &shellcmdflag, &shellredir] = [sh, shellcmdflag, shrd]
     if s:is_win && filereadable(batchfile)
@@ -2369,7 +2392,7 @@ endfunction
 
 function! s:system_chomp(...)
   let ret = call('s:system', a:000)
-  return v:shell_error ? '' : substitute(ret, '\n$', '', '')
+  return s:shell_error ? '' : substitute(ret, '\n$', '', '')
 endfunction
 
 function! s:git_validate(spec, check_branch)
@@ -2412,7 +2435,7 @@ function! s:git_validate(spec, check_branch)
           \ 'git', 'rev-list', '--count', '--left-right',
           \ printf('HEAD...origin/%s', origin_branch)
           \ ], a:spec.dir)), '\t')
-        if v:shell_error || len(ahead_behind) != 2
+        if s:shell_error || len(ahead_behind) != 2
           let err = "Failed to compare with the origin. The default branch might have changed.\nPlugClean required."
         else
           let [ahead, behind] = ahead_behind
@@ -2562,7 +2585,7 @@ function! s:upgrade()
 
   try
     let out = s:system(['git', 'clone', '--depth', '1', s:plug_src, tmp])
-    if v:shell_error
+    if s:shell_error
       return s:err('Error upgrading vim-plug: '. out)
     endif