raw_object_store: drop extra pointer to replace_map

raw_object_store: drop extra pointer to replace_map

We store the replacement data in an oidmap, which is itself a pointer in
the raw_object_store struct. But there's no need for an extra pointer
indirection here. It is always allocated and initialized along with the
containing struct, and we never check it for NULL-ness.

Let's embed the map directly in the struct, which is simpler and avoids
extra pointer chasing.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>