multi-pack-index: report progress during 'verify'

multi-pack-index: report progress during 'verify'

When verifying a multi-pack-index, the only action that takes
significant time is checking the object offsets. For example,
to verify a multi-pack-index containing 6.2 million objects in
the Linux kernel repository takes 1.3 seconds on my machine.
99% of that time is spent looking up object offsets in each of
the packfiles and comparing them to the multi-pack-index offset.

Add a progress indicator for that section of the 'verify' verb.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>