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>
diff --git a/parallel-checkout.c b/parallel-checkout.c
index 69d569f..2b1439c 100644
--- a/parallel-checkout.c
+++ b/parallel-checkout.c
@@ -8,6 +8,7 @@
 #include "parallel-checkout.h"
 #include "pkt-line.h"
 #include "progress.h"
+#include "read-cache-ll.h"
 #include "run-command.h"
 #include "sigchain.h"
 #include "streaming.h"