Shallow fetch for gitlab-foss

While debugging !264, there was a lot of CI noise (~4500 lines of CI logs) due to performing a git fetch for gitlab-foss, which by default pulls all tags, branches and commits. I believe we don't develop against this repository since it's a mirror with the ee/ folders stripped out.

If all we need this for is running tests against the latest commits, we can instead just do a shallow copy with only the master branch and the latest commit. This also puts less pressure on gitaly.