[GVFS Helper] Fix parallel load test

[derrickstolee]

@wilbaker noted in #122 that I had changed the parallel test to git rev-list --objects HEAD^{tree} and should not have. This makes the test do nothing of value: we already loaded all blobs at HEAD due to the non-sparse clone. (If we made the test do a sparse clone, then that would be interesting.)

However, without that change we get test failures in ParallelDownloadsInSharedCache().

After investigating, I think we have a different race condition that we didn't see before.

[derrickstolee]

I got the tests to pass locally on my Mac with the fix in microsoft/git#205.