Add failing test case for directory with ignored contents

[Minnozz]

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In git, the entire directory does not exist. In gitignore.nix, the empty directory does exist.

This illustrates issue #57.

[Minnozz]

I thought this test case would be the best way to illustrate a problem I'm experiencing with gitignore.nix.

[Minnozz]

❯ nix log /nix/store/7hdr9pfq4cg9j6hc4a0k3qfdlhai5wmk-plain.drv
49d48
< test-tree/13-dir-with-ignored-contents

Found a difference between nix-gitignore and native git.
Above diff can be read as a 'fix' to the nix-gitignore output.
< fix by excluding this in nix-gitignore
> fix by including this in nix-gitignore