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>

environment.c

@@ -17,6 +17,7 @@
 #include "commit.h"
 #include "strvec.h"
 #include "object-store.h"
+#include "replace-object.h"
 #include "tmp-objdir.h"
 #include "chdir-notify.h"
 #include "shallow.h"