Add a generic "object decorator" interface, and make object refs use it

This allows you to add an arbitrary "decoration" of your choice to any
object.  It's a space- and time-efficient way to add information to
arbitrary objects, especially if most objects probably do not have the
decoration.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

decorate.h

@@ -0,0 +1,18 @@
+#ifndef DECORATE_H
+#define DECORATE_H
+
+struct object_decoration {
+	struct object *base;
+	void *decoration;
+};
+
+struct decoration {
+	const char *name;
+	unsigned int size, nr;
+	struct object_decoration *hash;
+};
+
+extern void *add_decoration(struct decoration *n, struct object *obj, void *decoration);
+extern void *lookup_decoration(struct decoration *n, struct object *obj);
+
+#endif