quote: provide sq_dequote_to_argv_array

quote: provide sq_dequote_to_argv_array

This is similar to sq_dequote_to_argv, but more convenient
if you have an argv_array. It's tempting to just feed the
components of the argv_array to sq_dequote_to_argv instead,
but:

  1. It wouldn't maintain the NULL-termination invariant
     of argv_array.

  2. It doesn't match the memory ownership policy of
     argv_array (in which each component is free-able, not a
     pointer into a separate buffer).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>