Revert "Merge branch 'en/header-split-cache-h-part-3' into next"

This reverts commit 76ebce0b7a,
reversing changes made to 62e19d66fb,
in preparation for redoing the topic to make it buildable from the
source.

builtin/diff-index.c

@@ -1,13 +1,11 @@
-#include "builtin.h"
+#include "cache.h"
 #include "config.h"
 #include "diff.h"
 #include "diff-merges.h"
 #include "commit.h"
-#include "preload-index.h"
-#include "repository.h"
 #include "revision.h"
+#include "builtin.h"
 #include "setup.h"
-#include "sparse-index.h"
 #include "submodule.h"
 
 static const char diff_cache_usage[] =