Allow gitlab-project's fork-project command to fork projects between different repository storages

Closes #51

See merge request !75

Unset GL_ID and GL_PROTOCOL immediately

Neither other code executing in the hook process nor subprocesses of
the hook process need access to the GL_ID / GL_PROTOCOL variables so
we can just unset them the moment we fetch them.

See merge request !73

Neither other code executing in the hook process nor subprocesses of
the hook process need access to the GL_ID / GL_PROTOCOL variables so
we can just unset them the moment we fetch them.

Add an error message when using shell commands with incompatible GitLab versions

Closes #48

See merge request !70

Allow GitLab Shell to check for allowed access based on the used Git protocol.

Needed for gitlab-org/gitlab-ce!4696 and gitlab-org/gitlab-ce#18601

See merge request !62