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/combine-diff.c b/combine-diff.c
index 1a39b5d..91051dc 100644
--- a/combine-diff.c
+++ b/combine-diff.c
@@ -4,6 +4,7 @@
 #include "blob.h"
 #include "diff.h"
 #include "diffcore.h"
+#include "hex.h"
 #include "quote.h"
 #include "xdiff-interface.h"
 #include "xdiff/xmacros.h"