Create tags and branches directly using rugged instead of through gitlab-shell

Internal discussion: https://dev.gitlab.org/gitlab/gitlabhq/issues/2464

Right now, when you create or delete a tag or branch GitLab web UI, GitLab shells out to [gitlab-shell](https://gitlab.com/gitlab-org/gitlab-shell), which itself executes the git branch or git tag command in the repository directory.

We could make this a lot less complex by using Rugged instead, like we already do with most other Git commands, like creating files.