http: refactor subsystem to use packfile_lists

The dumb HTTP protocol directly fetches packfiles from the remote server
and temporarily stores them in a list of packfiles. Those packfiles are
not yet added to the repository's packfile store until we finalize the
whole fetch.

Refactor the code to instead use a `struct packfile_list` to store those
packs. This prepares us for a subsequent change where the `->next`
pointer of `struct packed_git` will go away.

Note that this refactoring creates some temporary duplication of code,
as we now have both `packfile_list_find_oid()` and `find_oid_pack()`.
The latter function will be removed in a subsequent commit though.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http.h

@@ -2,6 +2,7 @@
 #define HTTP_H
 
 struct packed_git;
+struct packfile_list;
 
 #include "git-zlib.h"
 
@@ -190,7 +191,7 @@ struct curl_slist *http_append_auth_header(const struct credential *c,
 
 /* Helpers for fetching packs */
 int http_get_info_packs(const char *base_url,
-			struct packed_git **packs_head);
+			struct packfile_list *packs);
 
 /* Helper for getting Accept-Language header */
 const char *http_get_accept_language_header(void);
@@ -226,7 +227,7 @@ void release_http_pack_request(struct http_pack_request *preq);
  * from http_get_info_packs() and have chosen a specific pack to fetch.
  */
 void http_install_packfile(struct packed_git *p,
-			   struct packed_git **list_to_remove_from);
+			   struct packfile_list *list_to_remove_from);
 
 /* Helpers for fetching object */
 struct http_object_request {