Alternate object pool mechanism updates.

Alternate object pool mechanism updates.

It was a mistake to use GIT_ALTERNATE_OBJECT_DIRECTORIES
environment variable to specify what alternate object pools to
look for missing objects when working with an object database.
It is not a property of the process running the git commands,
but a property of the object database that is partial and needs
other object pools to complete the set of objects it lacks.

This patch allows you to have $GIT_OBJECT_DIRECTORY/info/alternates
whose contents is in exactly the same format as the environment
variable, to let an object database name alternate object pools
it depends on.

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