cache.h: remove dependence on hex.h; make other files include it explicitly

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/diff.c b/diff.c
index 3c35659..ec911bd 100644
--- a/diff.c
+++ b/diff.c
@@ -9,6 +9,7 @@
 #include "diff.h"
 #include "diffcore.h"
 #include "delta.h"
+#include "hex.h"
 #include "xdiff-interface.h"
 #include "color.h"
 #include "attr.h"