commit-graph: check order while reading fanout chunk

commit-graph: check order while reading fanout chunk

We read the fanout chunk, storing a pointer to it, but only confirm that
the entries are monotonic in a final "lite" verification step. Let's
move that into the actual OIDF chunk callback, so that we can report
problems immediately (for all the reasons given in the previous
"commit-graph: abort as soon as we see a bogus chunk" commit).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>