object-store: move lookup_replace_object to replace-object.h

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>

object.c

@@ -1,5 +1,6 @@
 #include "cache.h"
 #include "object.h"
+#include "replace-object.h"
 #include "blob.h"
 #include "tree.h"
 #include "commit.h"