http: use off_t to store partial file size

When we try to resume transfer of a partially-downloaded object or pack, we fopen() the existing file for append, then use ftell() to get the current position. We use a "long", which can hold only 2GB on a 32-bit system, even though packfiles may be larger than that. Let's switch to using off_t, which should hold any file size our system is capable of storing. We need to use ftello() to get the off_t. This is in POSIX and hopefully available everywhere; if not, we should be able to wrap it by falling back to ftell(), which would presumably return "-1" on such a large file (and we would simply skip resuming in that case). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>