pack-redundant: gauge the usage before proposing its removal

pack-redundant: gauge the usage before proposing its removal

The subcommand is unusably slow and the reason why nobody reports it
as a performance bug is suspected to be the absense of users.  Let's
show a big message that asks the user to tell us that they still
care about the command when an attempt is made to run the command,
with an escape hatch to override it with a command line option.

In a few releases, we may turn it into an error and keep it for a
few more releases before finally removing it (during the whole time,
the plan to remove it would be interrupted by end user raising hand).

Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>