Geo: default gitlab-shell timeouts may be too short

ZD: https://gitlab.zendesk.com/agent/tickets/82309

Customer has a few large repositories that got killed due to the default 800 s timeout in git fetch passed by GitLab to gitlab-shell, resulting in lots of temporary pack files that for some reason never got cleaned up. We bumped up the timeout via:

gitlab_rails['git_timeout'] = 1800

This seems to have cut down the errors dramatically.

We should consider bumping up this default to a higher level for Geo--and possibly mirrors too.

/cc: @nick.thomas, @tiagonbotelho