index-pack: start learning to emulate "verify-pack -v"

index-pack: start learning to emulate "verify-pack -v"

The "index-pack" machinery already has almost enough knowledge to produce
the same output as "verify-pack -v". Fill small gaps in its bookkeeping,
and teach it to show what it knows.

Add a few more command line options that do not have to be advertised to
the end users. They will be used internally when verify-pack calls this.
The eventual goal is to remove verify-pack implementation and redo it as a
thin wrapper around the index-pack, so that we can remove the rather
expensive packed_object_info_detail() API.

This still does not do the delta-chain-depth histogram yet but that part
is easy.

Signed-off-by: Junio C Hamano <gitster@pobox.com>