Fix infinite recursion if .git directory doesn't exist

[TSRBerry]

This cost me a few hours yesterday when I was trying to figure out why my flake wasn't working anymore. Turns out all the files and directories of my gitignore (including the .git directory) were already filtered out by the time findGitIgnores runs.

This shouldn't be a problem as long as it eventually aborts, but that wasn't the case. Since each iteration adds another /.. to the path it can't ever be equal to / which causes the function to recurse forever.

By using dirOf instead of appending /.. we can avoid this issue and make sure this function always aborts at some point.

I'd like to add some tests for this, but I have no idea how to do that as I'm still new to Nix. Could you help me with that?

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