Avoid stub_gitlab_rb when testing git_data_dirs

stub_gitlab_rb makes sure the data is a Mash.

git_data_dirs({}) when part of gitlab.rb does not do that, so the test wasn't testing what it was intended to (it did not fail when reverting 542aea4a).

Here we bypass the use of our convenience helper in order to get an accurate reproduction of the issue so we can test the fix in 542aea4a

Fixup of !2307 (merged)