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.

Due to the way sshd works, gitlab-shell could be called with a single
string in the form:

```
/path/to/gitlab-shell -c key-id
```

However, due to the tightening of the regular expressions in fcff692b
this string no longer matches, so logins would fail with:

```
Failed to get username: who='' is invalid
```

This can be reproduced by changing the user's shell to point to
gitlab-shell. For example:

```
usermod git -s /opt/gitlab/embedded/service/gitlab-shell/bin/gitlab-shell
```

While setting gitlab-shell as the user's shell isn't officially
supported, gitlab-shell still should be able to cope with the key being
specified as the last argument. We now split the argument list and use
the last value.

Relates to https://gitlab.com/gitlab-org/gitlab-shell/-/issues/530

Use reflection to log the command we are about to execute, both in
gitlab-shell and gitlab-sshd. Include the environment, which has all
the context we need to understand what the command is expected to do.

Changelog: added

parse logic will only run if the executable accept args.
healthcheck is the only one not accepting arguments.

- We start supporting the "color" format for logs.
- We now respond to SIGHUP by reopening the log file.
- We now respect the log format when no log filename is specified.

Output to syslog in the event of logging system setup is preserved in
OpenSSH mode.

Changelog: added

Previously, opentracing (if configured) was initialized late in the
gitlab-shell process's lifespan, coming just before making a gRPC
call to Gitaly.

By moving the opentracing initialization to be at process startup, we
make it available for the whole process lifecycle, which is very useful
to gitlab-sshd, as it means we'll only call tracing.Initialize() once
on process startup, rather than once per SSH connection.

To get this working, we need to introduce a context to gitlab-sshd.
This carries the client/service name, but also carries an initial
correlation ID. The main outcome of this is that all calls to the
authorized_keys endpoint from a given gitlab-sshd process will now
share a correlation ID. I don't have a strong opinion about this either
way.

Changelog: fixed