argv-array: add detach function

argv-array: add detach function

The usual pattern for an argv array is to initialize it,
push in some strings, and then clear it when done. Very
occasionally, though, we must do other exotic things with
the memory, like freeing the list but keeping the strings.
Let's provide a detach function so that callers can make use
of our API to build up the array, and then take ownership of
it.

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