read_index_from(): speed index loading by skipping verification of the entry order

read_index_from(): speed index loading by skipping verification of the entry order

There is code in post_read_index_from() to catch out of order
entries when reading an index file.  This order verification is ~13%
of the cost of every call to read_index_from().

Update check_ce_order() so that it skips this verification unless
the "verify_ce_order" global variable is set.

Teach fsck to force this verification.

The effect can be seen using t/perf/p0002-read-cache.sh:

Test                                          HEAD              HEAD~1
--------------------------------------------------------------------------------------
0002.1: read_cache/discard_cache 1000 times   0.41(0.04+0.04)   0.50(0.00+0.10) +22.0%

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>