read-cache*.h: move declarations for read-cache.c functions from cache.h

For the functions defined in read-cache.c, move their declarations from
cache.h to a new header, read-cache-ll.h.  Also move some related inline
functions from cache.h to read-cache.h.  The purpose of the
read-cache-ll.h/read-cache.h split is that about 70% of the sites don't
need the inline functions and the extra headers they include.

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

diffcore-break.c

@@ -1,9 +1,10 @@
 /*
  * Copyright (C) 2005 Junio C Hamano
  */
-#include "cache.h"
+#include "git-compat-util.h"
 #include "diff.h"
 #include "diffcore.h"
+#include "object.h"
 #include "promisor-remote.h"
 
 static int should_break(struct repository *r,