If the GitLab API returns an allowed response with use_sidechannel set
to true, gitlab-shell will establish a sidechannel connection and use
SSHUploadPackWithSidechannel instead of SSHUploadPack. This is an
efficiency improvement.

This updates the Gitaly client go.mod dependency to Gitaly commit
2e398afa0490ccdf5a82e1a7c7d824ae491eba16. This causes a grpc-go
version bump, and hence a minor change in some of our test code.

This field will act as a feature flag that controls whether
gitlab-shell uses the old SSHUploadPack RPC or the new
SSHUploadPackWithSidechannel.

This reduces coupling between tests in
internal/gitlabnet/accessverifier/client_test.go, and will make it
easier to add new test cases in the future.

Note that the test server had a special behavior for the username
"second", but this was never used. So we removed that behavior in this
commit.

Deprecate self_signed_cert config setting

See merge request gitlab-org/gitlab-shell!552

The option isn't required to accept self-signed certs

On the other hand, if the option set to true it makes
machine-in-the-middle attack possible

Let's clarify it in the code that the option is deprecated

Suppress internal errors in client output

See merge request gitlab-org/gitlab-shell!549

Until recently, Gitaly was silently swallowing any errors returned by
SSH `git upload-pack` processes. Clients would still receive stderr
output and a non-zero return code, but Gitlab-Shell would receive error
as nil and log success.

With 9deaf47f1ecb00f0f36d18ee4a0fb1576f5a0efe Gitaly will now return an
error when git fails, but this causes Gitlab-Shell to print out the
GRPC error code as a message to the client:

> fatal: couldn't find remote ref not-a-real-ref
> fatal: the remote end hung up unexpectedly
> remote:
> remote:
> ========================================================================
> remote:
> remote: rpc error: code = Internal desc = SSHUploadPack: exit status 128
> remote:
> remote:
> ========================================================================
> remote:

The `remote:` text gives no additional context for the user and adds
clutter.

This commit suppresses the additional message added by Gitlab-Shell on
failure when the error type is `Internal`, returning client output to
the format it was prior to the Gitaly change.

Send full git request/response in SSHD tests

See merge request gitlab-org/gitlab-shell!550

Before 9deaf47f1ecb00f0f36d18ee4a0fb1576f5a0efe, Gitaly would return
success for `SSHUploadPack` and `SSHUploadArchive` regardless of the
exit code of the `git upload-pack|archive` process. As a result, the
gitlab-sshd acceptance tests could rely on no errors being returned from
Gitaly.

Currently these tests send the minimum request needed to start a
session, causing the server git process to fail as the `0000` flush
packet to end the session is never sent.

This commit fixes the tests by sending the full request/response needed
for a successful git operation.