This eliminates all use of byte-at-a-time reading of data in this
function: as Junio noted, a bundle file is seekable so we can
reset the file position to the first part of the pack-file using lseek
after reading the header.
Signed-off-by: Mark Levedahl <mdl123@verizon.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>