Newer versions of git (I've seen this behavior on 2.13.1.windows.1 and
have not seen it on 2.11.1.windows.1) are much slower at writing out local
copies of refs after a fetch. Given that we fetch about 10K refs when
running each PR job this can lead to a massive slowdown in fetch times (on
the roslyn integration job, which has a newer version of git on the
workers compared to the other machines, we see fetch taking ~20 mins).

This changes all the Roslyn jobs to fetch just the PR they are going to
test, which speeds up the fetch significantly.