fixup! gvfs-helper: run prefetch index-pack in parallel

[derrickstolee]

When adding the gvfs.prefetchThreads config option in #876, I didn't realize that the child git index-pack processes would output the hashes of the pack contents.

This makes the output quiet. It's committed as a fixup! so it can be squashed in the vfs-2.55.0 release (or this PR can be repeated on that branch and squashed in 2.56.0).

The tests check stderr because the parallel process runner sets stdout_to_stderr so the previous behavior showed up over stderr from the top level process.

• This change only applies to interactions with Azure DevOps and the
  GVFS Protocol.

[derrickstolee]

@dscho hopefully this is easy to cherry-pick into your rebase onto 2.54.0[.rc*]