When using the backup/restore task, GitLab copies the repositories
directory to a place like repositories.old.1493107724. This was
resulting in Git complaining about untracked files. Ignoring
everything starting with repositories solves that issue.

It's created when Runner with shell executor is used

This is a cause of problems for users. Although we already made fixing
it easy (gdk reconfigure) it was still hard for users to detect /
understand what was wrong.