Speed up spec/tasks/gitlab/git_rake_spec.rb

I noticed on https://redash.gitlab.com/dashboard/test-suite-statistics that this popped up as a slow spec (278 seconds).

Because no Git repository was actually created at the temporary path we were using, git fsck would traverse up until it found a repository, which in our case was the CE or EE repository.

Baseline:

Finished in 1 minute 35.91 seconds (files took 1.29 seconds to load)
3 examples, 0 failures

After:

Finished in 2.54 seconds (files took 1.52 seconds to load)
3 examples, 0 failures