git-daemon won't start up if its project directory (here /git) doesn't
exist. If we try to create it using the test harness, then we're racing
whether we manage to connect to the backdoor vs. the startup speed of
git-daemon.
Instead, use systemd-tmpfiles, which is guaranteed(?) to run before
network.target and thus before git-daemon.service starts.
This test is sometimes flaky on hydra as at the time of the `git clone`
the network isn't really configured yet[1]. That problem doesn't seem to
occur locally but if you run it on a machine with high enough load (such
as hydra build machines). Hopefully this will make the test not flaky
anymore.
[1] https://hydra.nixos.org/build/118710378/nixlog/21/raw
