repository.h: move declaration of the_index from cache.h

the_index is a global variable defined in repository.c; as such, its
declaration feels better suited living in repository.h rather than
cache.h.  Move it.

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

t/helper/test-dump-split-index.c

@@ -2,6 +2,7 @@
 #include "test-tool.h"
 #include "cache.h"
 #include "hex.h"
+#include "repository.h"
 #include "setup.h"
 #include "split-index.h"
 #include "ewah/ewok.h"