fetch: use argv_array instead of hand-building arrays

fetch: use argv_array instead of hand-building arrays

Fetch invokes itself recursively when recursing into
submodules or handling "fetch --multiple". In both cases, it
builds the child's command line by pushing options onto a
statically-sized array. In both cases, the array is
currently just big enough to handle the largest possible
case. However, this technique is brittle and error-prone, so
let's replace it with a dynamic argv_array.

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