tsort: returns error when input is dir - same as GNU tsort

[SaHHiiLL]

fix: #5853

[cakebaker]

Can you please add a test so we don't regress in the future?

[SaHHiiLL]

sure!

[SaHHiiLL]

lmk if there's something else I can put instead of . for the test

[cakebaker]

I think . is fine.

If you want to specify a concrete directory name, you can use something like:

let (at, mut ucmd) = at_and_ucmd!();
at.mkdir("dirname");

And then use ucmd instead of new_ucmd!.

[SaHHiiLL]

Alright, That seems to work. Thanks for the help!!

[cakebaker]

Thanks for your PR :)