Vim: use close_cb to trigger exit_cb

Before Vim 8.0.50, an exited job was only detected with a potentially huge
delay.

Thus, for versions smaller than 8.0.50, we add a close_cb callback that runs
job_status() which in return runs the exit_cb callback immeditely if the job was
found to be dead.

Vim patch:

    01688ad545

References: #216.

autoload/sy/repo.vim

@@ -36,6 +36,14 @@ function! s:callback_stdout_vim(_job_id, data) dict abort
   let self.stdoutbuf += [a:data]
 endfunction
 
+" Function: s:callback_close {{{1
+function! s:callback_close(channel) dict abort
+  try
+    silent! call job_status(ch_getjob(a:channel))
+  catch
+  endtry
+endfunction
+
 " Function: s:callback_exit {{{1
 function! s:callback_exit(_job_id, exitval, ...) dict abort
   call sy#verbose('callback_exit()', self.vcs)
@@ -83,11 +91,15 @@ function! sy#repo#get_diff_start(vcs, do_register) abort
 
     try
       execute chdir fnameescape(b:sy_info.dir)
-      let b:sy_job_id_{a:vcs} = job_start(cmd, {
+      let opts = {
             \ 'in_io':   'null',
             \ 'out_cb':  function('s:callback_stdout_vim', options),
             \ 'exit_cb': function('s:callback_exit', options),
-            \ })
+            \ }
+      if !has('patch-8.0.50')
+        let opts.close_cb = function('s:callback_close')
+      endif
+      let b:sy_job_id_{a:vcs} = job_start(cmd, opts)
     finally
       execute chdir fnameescape(cwd)
     endtry

doc/signify.txt

@@ -567,7 +567,7 @@ The plugin is slow!~
 Line highlighting without showing signs?~
 
 The line highlighting relies on signs being placed. The sign column is being
-shown automatically when there are placed signs.
+shown automatically if there are placed signs.
 
 With a recent Vim, you can change that behaviour using 'signcolumn'.