Add support for external programs for handling native fetches

Add support for external programs for handling native fetches

transport_get() can call transport_native_helper_init() to have list and
fetch-ref operations handled by running a separate program as:

 git remote-<something> <remote> [<url>]

This program then accepts, on its stdin, "list" and "fetch <hex>
<name>" commands; the former prints out a list of available refs and
either their hashes or what they are symrefs to, while the latter
fetches them into the local object database and prints a newline when done.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>