Be careful when dereferencing tags.

One caller of deref_tag() was not careful enough to make sure
what deref_tag() returned was not NULL (i.e. we found a tag
object that points at an object we do not have).  Fix it, and
warn about refs that point at such an incomplete tag where
needed.

Signed-off-by: Junio C Hamano <junkio@cox.net>

name-rev.c

@@ -164,7 +164,7 @@ int main(int argc, char **argv)
 			continue;
 		}
 
-		o = deref_tag(parse_object(sha1));
+		o = deref_tag(parse_object(sha1), *argv, 0);
 		if (!o || o->type != commit_type) {
 			fprintf(stderr, "Could not get commit for %s. Skipping.\n",
 					*argv);