http-fetch: support fetching packfiles by URL

http-fetch: support fetching packfiles by URL

Teach http-fetch the ability to download packfiles directly, given a
URL, and to verify them.

The http_pack_request suite has been augmented with a function that
takes a URL directly. With this function, the hash is only used to
determine the name of the temporary file.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>