Fix up recent object model cleanups

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

Makefile

@@ -27,7 +27,7 @@ install: $(PROG) $(SCRIPTS)
 
 LIB_OBJS=read-cache.o sha1_file.o usage.o object.o commit.o tree.o blob.o tag.o
 LIB_FILE=libgit.a
-LIB_H=cache.h object.h
+LIB_H=cache.h object.h blob.h tree.h commit.h tag.h
 
 LIB_H += strbuf.h
 LIB_OBJS += strbuf.o

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 */