Extract verify_pack_index for reuse from verify_pack
The dumb HTTP transport should verify an index is completely valid
before trying to use it. That requires checking the header/footer
but also checking the complete content SHA-1. All of this logic is
already in the front half of verify_pack, so pull it out into a new
function that can be reused.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>