tutorial: setting up a tree for subsystem maintainers

tutorial: setting up a tree for subsystem maintainers

The "copying over packs" step is to prevent the objects
available in upstream repository to get expanted in the
subsystem maintainer tree, and is still valid if the upstream
repository do not live on the same machine.  But if they are on
the same machine using objects/info/alternates is cleaner.

Signed-off-by: Junio C Hamano <junkio@cox.net>