Make GitFileSystemObjectSink multi-threaded

[edolstra]

Motivation

This speeds up importing tarballs into the Git cache.

Time for importing a nixpkgs tarballs on Nix master:

# rm -rf ~/.cache/nix/tarball-cache/; command time nix flake metadata tarball+file:///.../master.tar.gz
20.00user 2.41system 0:12.13elapsed 184%CPU (0avgtext+0avgdata 480952maxresident)k
439396inputs+462197outputs (13910major+163516minor)pagefaults 0swaps

With this PR:

4.59user 9.37system 0:03.68elapsed 379%CPU (0avgtext+0avgdata 229920maxresident)k
499429inputs+473245outputs (0major+82815minor)pagefaults 0swaps

TODO: this currently buffers all file contents in memory before writing them to disk, so it's no longer constant memory. But this could be fixed pretty easily (e.g. if there are more than N bytes worth of unwritten files, then wait).

Context

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