How can the cmd window be hidden?

Hi, all.
I am using gnu global with gvim on windows 7.
It woks fine and but a small problem bothes me.

I write the following code in vimrc:

au BufWritePost *.[ch],*.cpp call UpdateGtags(expand('<afile>'))
function! UpdateGtags(f)
let dir = fnamemodify(a:f, ':p:h')
exe 'silent !cd ' . dir . ' && global -u &> /dev/null &'
endfunction

But when I try to save content to gvim, a black window of cmd blinks,
and disappears.
I know it's vim calls it.

I want to let the cmd window run in the background. How can I do it?

Thanks in advance.

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