lookup_replace_object is a low-level function that most users of the
object store do not need to use directly.
Move it to replace-object.h to avoid a dependency loop in an upcoming
change to its inline definition that will make use of repository.h.
Signed-off-by: Stefan Beller <sbeller@google.com> Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>