Given the gitaly-* now proxy the data from the client to the Gitaly
server, the environment variables aren't used. Therefor we don't have to
set them either. Only exception to the rule, is the GITALY_TOKEN.

These changes also remove the `GIT_TRACE` options, introduced by
192e2bd367494bf66746c8971896a2d9cb84fc92.

Part of: https://gitlab.com/gitlab-org/gitaly/issues/1300

All shell access goes through Gitaly, so dead code paths exist to
support the legacy way too.

This change mostly removes the dead code from `#process_cmd`.

Remove repo_path from GitlabShell

Closes #135

See merge request gitlab-org/gitlab-shell!231

The internal api returns '/' from gitlab, since
`8fad07383ada021fc995294fd0fe0f77fe37da35` from GitLab CE. To clean up
later, https://gitlab.com/gitlab-org/gitlab-shell/issues/135 was
created.

This change closes that issue, making it possible to remove the field
from the response on GitLab-CE too. Given the Rails app always returns
`/` as the repository_path, the associated checks are basically a noop
too. The tests are updated and at times look a little fishy, but those
are testing code that is to be removed in another MR.

Closes https://gitlab.com/gitlab-org/gitlab-shell/issues/135

Update to RSpec 3

Closes #54

See merge request gitlab-org/gitlab-shell!229