This is primarily useful for version string generation, where we need
a monotonically increasing number. The revcount is the preferred thing
to use, but isn't available for GitHub flakes (since it requires
fetching the entire history). The last commit timestamp OTOH can be
extracted from GitHub tarballs.
Unlike file://<path>, this allows the path to be a dirty Git tree, so
nix build /path/to/flake:attr
is a convenient way to test building a local flake.