Add mq status for hg repositories

Now that we have async feature for Vim 8, we can add the output of 'hg
qtop' back to the statusline if it is supported

autoload/airline/extensions/branch.vim

@@ -106,7 +106,18 @@ endfunction
 
 function! s:get_hg_branch()
   if s:has_lawrencium
-    return lawrencium#statusline()
+    let stl=lawrencium#statusline()
+    if !empty(stl) && has('job')
+      call s:get_mq_async('hg qtop', expand('%:p'))
+    endif
+    if exists("s:mq") && !empty(s:mq)
+      if stl is# 'default'
+        " Shorten default a bit
+        let stl='def'
+      endif
+      let stl.=' ['.s:mq.']'
+    endif
+    return stl
   endif
   return ''
 endfunction
@@ -154,6 +165,44 @@ if s:has_async
           \ 'close_cb': function('s:on_exit', options)})
     let s:jobs[a:file] = id
   endfu
+
+  function! s:on_exit_mq(channel) dict abort
+    if !empty(self.buf)
+      if self.buf is# 'no patches applied' ||
+        \ self.buf =~# "unknown command 'qtop'"
+        let self.buf = ''
+      elseif exists("s:mq") && s:mq isnot# self.buf
+        " make sure, statusline is updated
+        unlet! b:airline_head
+      endif
+      let s:mq = self.buf
+    endif
+    if has_key(s:jobs, self.file)
+      call remove(s:jobs, self.file)
+    endif
+  endfunction
+
+  function! s:get_mq_async(cmd, file)
+    if g:airline#util#is_windows && &shell =~ 'cmd'
+      let cmd = a:cmd. shellescape(a:file)
+    else
+      let cmd = ['sh', '-c', a:cmd]
+    endif
+
+    let options = {'cmd': a:cmd, '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_mq', options)})
+    let s:jobs[a:file] = id
+  endfu
 endif
 
 function! airline#extensions#branch#head()