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.

strbuf.h

@@ -72,6 +72,10 @@ struct strbuf {
 extern char strbuf_slopbuf[];
 #define STRBUF_INIT  { .buf = strbuf_slopbuf }
 
+/*
+ * Predeclare this here, since cache.h includes this file before it defines the
+ * struct.
+ */
 struct object_id;
 
 /**