Restore functionality to allow proxies to cache objects

Restore functionality to allow proxies to cache objects

The parallel request changes didn't properly implement the previous patch to
allow caching of retrieved objects by proxy servers.  Restore the previous
functionality such that by default requests include the "Pragma: no-cache"
header, and this header is removed on requests for pack indexes, packs, and
objects.

Signed-off-by: Nick Hengeveld <nickh@reactrix.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>