argv-array: add pop function

argv-array: add pop function

Sometimes we build a set of similar command lines, differing
only in the final arguments (e.g., "fetch --multiple"). To
use argv_array for this, you have to either push the same
set of elements repeatedly, or break the abstraction by
manually manipulating the array's internal members.

Instead, let's provide a sanctioned "pop" function to remove
elements from the end.

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