http*: add helper methods for fetching packs

http*: add helper methods for fetching packs

The code handling the fetching of packs in http-push.c and
http-walker.c have been refactored into new methods and a new struct
(http_pack_request) in http.c. They are not meant to be invoked
elsewhere.

The new methods in http.c are
 - new_http_pack_request
 - finish_http_pack_request
 - release_http_pack_request

and the new struct is http_pack_request.

Add a function, new_http_pack_request(), that deals with the details of
coming up with the filename to store the retrieved packfile, resuming a
previously aborted request, and making a new curl request. Update
http-push.c::start_fetch_packed() and http-walker.c::fetch_pack() to
use this.

Add a function, finish_http_pack_request(), that deals with renaming
the pack, advancing the pack list, and installing the pack. Update
http-push.c::finish_request() and http-walker.c::fetch_pack to use
this.

Update release_request() in http-push.c and http-walker.c to invoke
release_http_pack_request() to clean up pack request helper data.

The local_stream member of the transfer_request struct in http-push.c
has been removed, as the packfile pointer will be managed in the struct
http_pack_request.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>