SendArchive rejects '.tgz' extension

Although the GitLab web UI does not offer an 'archive.tgz' button, the API does allow that format. But we reject it in git.archive.Inject.

List of formats: https://gitlab.com/gitlab-org/gitlab-ce/blob/065341bf04958b862db9cc2891c7822a3da7f7f8/lib/gitlab/regex.rb#L66-69