async: fnameescape filename

A buffer name wich special characters such as Command-T [Files] will
cause E94 when trying to call setbufvar() on it. That is because the
'[]' is special in buffer names. Therefore, fnameescape the buffer name.

This should prevent 'E94: No matching buffer for
/home/louis/git/minivimrc/Command-T [Files]' errors.

closes #1906

autoload/airline/async.vim

@@ -67,9 +67,9 @@ endfunction
 function! s:on_exit_clean(...) dict abort
   let buf=self.buf
   if !empty(buf)
-    let var=getbufvar(self.file, 'buffer_vcs_config', {})
+    let var=getbufvar(fnameescape(self.file), 'buffer_vcs_config', {})
     let var[self.vcs].dirty=1
-    call setbufvar(self.file, 'buffer_vcs_config', var)
+    call setbufvar(fnameescape(self.file), 'buffer_vcs_config', var)
     unlet! b:airline_head
   endif
   if has_key(get(s:clean_jobs, 'self.vcs', {}), self.file)