This fixes a regression caused by commit f56ab9e
("nix-prefetch-git: Include the date in the machine-readable [...]")
where a couple of directory paths printed by pushd/popd appeared before
the JSON output on stdout (thus breaking it). Fix it by redirecting the
extraneous output to /dev/null.
Reported by Michael Alan Dorman <mdorman@ironicdesign.com>.
