replace-object: move replace_map to object store

replace-object: move replace_map to object store

The relationship between an object X and another object Y that
replaces the object X is defined only within the scope of a
single repository.

The exception in reachability rule around these replacement objects
is also local to a repository (i.e. if traversal from refs reaches
X, then both X and Y are reachable and need to be kept from gc).

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>