treewide: reduce includes of cache.h in other headers

We had a handful of headers including cache.h that didn't need to
anymore.  Drop those includes and replace them with includes of
smaller files, or forward declarations.  However, note that two .c
files now need to directly include cache.h, though they should have
been including it all along given they are directly using structs
defined in it.

Signed-off-by: Elijah Newren <newren@gmail.com>
Acked-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

split-index.h

@@ -1,7 +1,7 @@
 #ifndef SPLIT_INDEX_H
 #define SPLIT_INDEX_H
 
-#include "cache.h"
+#include "hash.h"
 
 struct index_state;
 struct strbuf;