preload-index.h: move declarations for preload-index.c from elsewhere

We already have a preload-index.c file; move the declarations for the
functions in that file into a new preload-index.h.  These were
previously split between cache.h and repository.h.

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

builtin/diff-files.c

@@ -8,6 +8,7 @@
 #include "diff.h"
 #include "diff-merges.h"
 #include "commit.h"
+#include "preload-index.h"
 #include "revision.h"
 #include "builtin.h"
 #include "submodule.h"