Make "tree_entry" have a SHA1 instead of a union of object pointers

This is preparatory work for further cleanups, where we try to make
tree_entry look more like the more efficient tree-walk descriptor.

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

fsck-objects.c

@@ -464,6 +464,7 @@ int main(int argc, char **argv)
 {
 	int i, heads;
 
+	track_object_refs = 1;
 	setup_git_directory();
 
 	for (i = 1; i < argc; i++) {