[PATCH] Dereference tag repeatedly until we get a non-tag.

When we allow a tag object in place of a commit object, we only
dereferenced the given tag once, which causes a tag that points at a tag
that points at a commit to be rejected.  Instead, dereference tag
repeatedly until we get a non-tag.

This patch makes change to two functions:

 - commit.c::lookup_commit_reference() is used by merge-base,
   rev-tree and rev-parse to convert user supplied SHA1 to that of
   a commit.
 - rev-list uses its own get_commit_reference() to do the same.

Dereferencing tags this way helps both of these uses.

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

commit.c

@@ -52,8 +52,9 @@ struct commit *lookup_commit_reference(const unsigned char *sha1)
 
 	if (!obj)
 		return NULL;
-	if (obj->type == tag_type)
-		obj = ((struct tag *)obj)->tagged;
+	while (obj->type == tag_type)
+		obj = parse_object(((struct tag *)obj)->tagged->sha1);
+
 	return check_commit(obj, sha1);
 }