Fix handling non 200 response codes

Closes #152

See merge request gitlab-org/gitlab-shell!235

After the cleanup in https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/231
gitlab-shell mishandled the non 200 response codes. This commit removes
another few lines of codes, which fixes this. Also now we test against
this case through mocking.

Fixes https://gitlab.com/gitlab-org/gitlab-shell/issues/152

Cleanup `git-upload-*` and `git-receive-*` related code

Closes gitaly#1300

See merge request gitlab-org/gitlab-shell!232

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
192e2bd3.

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

This defines `ROOT_PATH`, which we later stub.

Resolve "gitlab-shell 8.1.1 not on master"

Closes #148 and #145

See merge request gitlab-org/gitlab-shell!230

Revert refactor

Closes #147

See merge request gitlab-org/gitlab-shell!228

This reverts commit 764f6f47.

This reverts commit 3aaf4751, reversing
changes made to c6577e0d.

This reverts commit f4ce4a3c.

Fix two regressions in SSH certificate support (8.1)

See merge request gitlab-org/gitlab-shell!226

Add missing require statement in create-hooks

Closes gitlab-ce#50160

See merge request gitlab-org/gitlab-shell!225

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/50160

Pass custom git_config_options to Gitaly

See merge request gitlab-org/gitlab-shell!221

Fix two regressions in my 2e8b6702 ("Add support for SSH certificate
authentication", 2018-06-14) merged in gitlab-org/gitlab-shell!207.

This fixes the issue noted in gitlab-org/gitlab-shell#145 where the
command-line contains things other than the key/user/username, and
also a regression where SSH certificates are being used, and the
username presented in the key is unknown to GitLab.

In that case, we should log the user in as "Anonymous" (on an instance
that allows public access), but because of how the error checking
around api.discover() was implemented we ended up erroring out
instead.

Refactor that focuses on SRP improvements

See merge request gitlab-org/gitlab-shell!214

The /api/internal/allowed endpoint on GitLab has been updated to return some
custom git options that can be used with git commands, we need to pass these received options to Gitaly.