Use neovim's async job's instead of system

This commit makes branch.vim use neovim's async jobs instead of a system()
function. This way we avoid the v:shell_error overwrite bug and allow live
updates of the untracked status.

autoload/airline/util.vim

@@ -77,3 +77,34 @@ else
     return 0
   endfunction
 endif
+
+" Define a wrapper over system() that uses nvim's async job control if
+" available. This way we avoid overwriting v:shell_error, which might
+" potentially disrupt other plugins.
+if has('nvim')
+  function! s:system_job_handler(job_id, data, event)
+    if a:event == 'stdout'
+      let self.buf .=  join(a:data)
+    endif
+  endfunction
+
+  function! airline#util#system(cmd)
+    let l:config = {
+    \ 'buf': '',
+    \ 'on_stdout': function('s:system_job_handler'),
+    \ }
+    let l:id = jobstart(a:cmd, l:config)
+    if l:id < 1
+      return system(a:cmd)
+    endif
+    let l:ret_code = jobwait([l:id])
+    if l:ret_code != [0]
+      return system(a:cmd)
+    endif
+    return l:config.buf
+  endfunction
+else
+  function! airline#util#system(cmd)
+    return system(a:cmd)
+  endfunction
+endif