Parse messages from guru on close_cb, not exit_cb

There is a race condition in `async_guru` in which not all messages from
a `guru` call are displayed:

1. An async `guru` call is started.

2. Lines streamed from `guru` are appended to a message list in
   `callback()`.

3. The `guru` process ends, `exit_cb()` is called, and the list of
   messages is processed for display.

4. The rest of the lines from `guru` are appended to the message list.

Note that `callback` may still be called after `exit_cb`, as noted in
`:help exit_cb`:

	"exit_cb": handler	…
				Note that data can be buffered,
				callbacks may still be called after
				the process ends.

This race was recently exposed by [Vim v8.0.1073][vim], which avoids a
redraw after statusline evaluation changes a highlight. Since vim-go
modifies the goStatusLineColor highlight definition on async calls, the
corresponding redraw acted as a delay, masking this data race.

The solution to this problem is to process `guru` output in `close_cb`,
which is called after all `callback` calls are complete.

[vim]: vim/vim@ba2929b

autoload/go/guru.vim

@@ -159,31 +159,38 @@ function! s:async_guru(args) abort
     call add(messages, a:msg)
   endfunction
 
-  function! s:exit_cb(job, exitval) closure
-    let out = join(messages, "\n")
+  let status = {}
+  let exitval = 0
 
+  function! s:exit_cb(job, exitval) closure
     let status = {
           \ 'desc': 'last status',
           \ 'type': statusline_type,
           \ 'state': "finished",
           \ }
 
     if a:exitval
+      let exitval = a:exitval
       let status.state = "failed"
     endif
 
     call go#statusline#Update(status_dir, status)
+  endfunction
+
+  function! s:close_cb(ch) closure
+    let out = join(messages, "\n")
 
     if has_key(a:args, 'custom_parse')
-      call a:args.custom_parse(a:exitval, out)
+      call a:args.custom_parse(exitval, out)
     else
-      call s:parse_guru_output(a:exitval, out, a:args.mode)
+      call s:parse_guru_output(exitval, out, a:args.mode)
     endif
   endfunction
 
   let start_options = {
         \ 'callback': funcref("s:callback"),
         \ 'exit_cb': funcref("s:exit_cb"),
+        \ 'close_cb': funcref("s:close_cb"),
         \ }
 
   if has_key(result, 'stdin_content')