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/notes-merge.c b/notes-merge.c
index b4cc594..5b1a9ff 100644
--- a/notes-merge.c
+++ b/notes-merge.c
@@ -5,6 +5,7 @@
 #include "repository.h"
 #include "diff.h"
 #include "diffcore.h"
+#include "hex.h"
 #include "xdiff-interface.h"
 #include "ll-merge.h"
 #include "dir.h"