scalar: teach `diagnose` to gather loose objects information

scalar: teach `diagnose` to gather loose objects information

When operating at the scale that Scalar wants to support, certain data
shapes are more likely to cause undesirable performance issues, such as
large numbers of loose objects.

By including statistics about this, `scalar diagnose` now makes it
easier to identify such scenarios.

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>