This is based on @eighthave's templates, and the work we've been
doing to present a uniform testing environment.

Adjust the GitLab CI configuration to more closely match that of
Travis CI. Fixes bug 23757.

Stop attempting to unconditionally mirror the tor repository in GitLab
CI.  This prevented developers from enabling GitLab CI on master
because the "update" job would attempt to run, causing an unuseful CI
failure.  Fixes bug 23755.