hush: reinstate [[ builtin

hush: reinstate [[ builtin

Mike deleted it:

    commit 39456a18a104b228de240b265bd943251219849d
    Author: Mike Frysinger <vapier@gentoo.org>
    Date:   Sat Mar 28 12:21:57 2009 +0000

    stop lying about [[ test support

probably because it was not properly ifdefed around, and was enabled
even when bash compat is off.

I just tested it - it works:

$ [ *.diff = z.diff ]; echo $?
0
$ [[ *.diff = z.diff ]]; echo $?
1

Of course, not all numerous bash tricks of [[ ]] are implemented...

function                                             old     new   delta
bltins2                                               60      72     +12

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>