Add message attribute, lookup using Rugged lib, to Gitlab::Git::Tag.

For regular (lightweight) tags I set `message = nil`, which should be the previous behavior, but for annotated tags `message` will be set appropriately.  The idea being that the message should be returned when doing operations via the API and can also be displayed on the "Git tags" page.

I noticed there's no test for the `Gitlab::Git::Repository#tags` method.  I don't have a problem writing the test however I might need someone to add at least 1 annotated tag to the seed repo: https://gitlab.com/gitlab-org/gitlab-git-test.git.

As always, open to feedback.