index-pack: use fixup_pack_header_footer()'s validation mode

index-pack: use fixup_pack_header_footer()'s validation mode

When completing a thin pack, a new header has to be written to
the pack and a new SHA1 computed.  Make sure that the SHA1 of what
is being read back matches the SHA1 of what was written for both:
the original pack and the appended objects.

To do so, a couple write_or_die() calls were converted to sha1write()
which has the advantage of doing some buffering as well as handling
SHA1 and CRC32 checksum already.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>