Issue description

Currently, using gitTracked ./. in a local project fails when that project is fetched with e.g. fetchGit, since there's no .git directory in that case.

Steps to reproduce

With https://github.com/infinisil/gitTracked-example/blob/main/default.nix:

$ nix-instantiate https://github.com/infinisil/gitTracked-example/tarball/master --eval -A outPath
error: lib.fileset.gitTracked: Expected the argument (/nix/store/sdhphiawzjiw9yxr50iqf729sd14wmy1-source) to point to a local working tree of a Git repository, but it's not.

Proposed solution

Detect whether the passed path is already in the Nix store, in which case just return the set of all its files. Only fail if that's not the case.
A bit icky but might be fine? Needs a bit more thought.

This issue is sponsored by Antithesis ✨