Fix up recent object model cleanups

Make sure the Makefile knows about the object header dependencies, and
add declarations for tag lookup/parsing.

tag.h

@@ -12,4 +12,7 @@ struct tag {
 	char *signature; /* not actually implemented */
 };
 
+extern struct tag *lookup_tag(unsigned char *sha1);
+extern int parse_tag(struct tag *item);
+
 #endif /* TAG_H */