git-describe: use tags found in packed-refs correctly

When your refs are packed, "git-describe" can find the tag that is the
best match without ever parsing the tag itself.  But lookup_tag() in
display_name() says "I've never seen it", creates an empty shell, and
returns it.  We need to make sure that we actually have parsed the tag
data into it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-describe.c

@@ -156,7 +156,7 @@ static void display_name(struct commit_name *n)
 {
 	if (n->prio == 2 && !n->tag) {
 		n->tag = lookup_tag(n->sha1);
-		if (!n->tag || !n->tag->tag)
+		if (parse_tag(n->tag) || !n->tag || !n->tag->tag)
 			die("annotated tag %s not available", n->path);
 		if (strcmp(n->tag->tag, n->path))
 			warning("tag '%s' is really '%s' here", n->tag->tag, n->path);