midx: keep track of the checksum
write_midx_internal() uses a hashfile to write the multi-pack index, but
discards its checksum. This makes sense, since nothing that takes place
after writing the MIDX cares about its checksum.
That is about to change in a subsequent patch, when the optional
reverse index corresponding to the MIDX will want to include the MIDX's
checksum.
Store the checksum of the MIDX in preparation for that.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>