Refactor async functions

- create a new async module
- refactor async functions from branch.vim and po.vim to async.vim
- support nvim async correctly

autoload/airline/extensions/po.vim

@@ -3,7 +3,7 @@
 
 scriptencoding utf-8
 
-function! s:shorten()
+function! airline#extensions#po#shorten()
   if exists("g:airline#extensions#po#displayed_limit")
     let w:displayed_po_limit = g:airline#extensions#po#displayed_limit
     if len(b:airline_po_stats) > w:displayed_po_limit - 1
@@ -12,49 +12,6 @@ function! s:shorten()
   endif
 endfunction
 
-if g:airline#init#async
-  let s:jobs = {}
-
-  function! s:on_stdout(channel, msg) dict abort
-    let self.buf = a:msg
-  endfunction
-
-  function! s:on_exit(channel) dict abort
-    if !empty(self.buf)
-      let b:airline_po_stats = printf("[%s]", self.buf)
-    else
-      let b:airline_po_stats = ''
-    endif
-    if has_key(s:jobs, self.file)
-      call remove(s:jobs, self.file)
-    endif
-    call s:shorten()
-  endfunction
-
-  function! s:get_msgfmt_stat_async(cmd, file)
-    if g:airline#init#is_windows || !executable('msgfmt')
-      " no msgfmt on windows?
-      return
-    else
-      let cmd = ['sh', '-c', a:cmd. shellescape(a:file)]
-    endif
-
-    let options = {'buf': '', 'file': a:file}
-    if has_key(s:jobs, a:file)
-      if job_status(get(s:jobs, a:file)) == 'run'
-        return
-      elseif has_key(s:jobs, a:file)
-        call remove(s:jobs, a:file)
-      endif
-    endif
-    let id = job_start(cmd, {
-          \ 'err_io':   'out',
-          \ 'out_cb':   function('s:on_stdout', options),
-          \ 'close_cb': function('s:on_exit', options)})
-    let s:jobs[a:file] = id
-  endfu
-endif
-
 function! airline#extensions#po#apply(...)
   if &ft ==# 'po'
     call airline#extensions#prepend_to_section('z', '%{airline#extensions#po#stats()}')
@@ -69,7 +26,7 @@ function! airline#extensions#po#stats()
 
   let cmd = 'msgfmt --statistics -o /dev/null -- '
   if g:airline#init#async
-    call s:get_msgfmt_stat_async(cmd, expand('%:p'))
+    call airline#async#get_msgfmt_stat(cmd, expand('%:p'))
   else
     let airline_po_stats = system(cmd. shellescape(expand('%:p')))
     if v:shell_error
@@ -81,7 +38,7 @@ function! airline#extensions#po#stats()
     catch
       let b:airline_po_stats = ''
     endtry
-    call s:shorten()
+    call airline#extensions#po#shorten()
   endif
   return get(b:, 'airline_po_stats', '')
 endfunction