replace-object.c: remove the_repository from prepare_replace_object

replace-object.c: remove the_repository from prepare_replace_object

This was missed in 5982da9d2ce (replace-object: allow
prepare_replace_object to handle arbitrary repositories, 2018-04-11)

Technically the code works correctly as the replace_map is the same
size in different repositories, however it is hard to read. So convert
the code to the familiar pattern of dereferencing the pointer that we
assign in the sizeof itself.

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