http*: add http_get_info_packs

http-push.c and http-walker.c no longer have to use fetch_index or
setup_index; they simply need to use http_get_info_packs, a new http
method, in their fetch_indices implementations.

Move fetch_index() and rename to fetch_pack_index() in http.c; this
method is not meant to be used outside of http.c. It invokes
end_url_with_slash with base_url; apart from that change, the code is
identical.

Move setup_index() and rename to fetch_and_setup_pack_index() in
http.c; this method is not meant to be used outside of http.c.

Do not immediately set ret to 0 in http-walker.c::fetch_indices();
instead do it in the HTTP_MISSING_TARGET case, to make it clear that
the HTTP_OK and HTTP_MISSING_TARGET cases both return 0.

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

http.h

@@ -146,4 +146,8 @@ int http_error(const char *url, int ret);
 
 extern int http_fetch_ref(const char *base, struct ref *ref);
 
+/* Helpers for fetching packs */
+extern int http_get_info_packs(const char *base_url,
+	struct packed_git **packs_head);
+
 #endif /* HTTP_H */