replace-object.h: move read_replace_refs declaration from cache.h to here

Adjust several files to be more explicit about their dependency on
replace-objects to accommodate this change.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/unpack-objects.c

@@ -9,6 +9,7 @@
 #include "pack.h"
 #include "blob.h"
 #include "commit.h"
+#include "replace-object.h"
 #include "tag.h"
 #include "tree.h"
 #include "tree-walk.h"