Initial Windows support for msgfmt

For now, it is tested successfully on the latest Vim 8.2 running on Windows 10.
Needs further change and testing for Neovim and older Vim versions.

autoload/airline/async.vim

@@ -153,9 +153,12 @@ if v:version >= 800 && has("job")
   endfunction
 
   function! airline#async#get_msgfmt_stat(cmd, file)
-    if g:airline#init#is_windows || !executable('msgfmt')
-      " no msgfmt on windows?
+    if !executable('msgfmt')
+      " no msgfmt
       return
+    endif
+    if g:airline#init#is_windows
+      let cmd = 'cmd /C ' . a:cmd. shellescape(a:file)
     else
       let cmd = ['sh', '-c', a:cmd. shellescape(a:file)]
     endif