fix: Throw an error when readdir is called on a non-directory

[AtkinsSJ]

This replaces a72ec97 as a fix for #417.

That change meant that calling readdir on a file path would return a single stat entry, for that file. The problem is, to callers that looks like the file is a directory containing itself. This was causing problems in isomorphic-git, as it would readdir foo.txt, receive the listing of [ {path: foo.txt', ... } ], and then try to read foo.txt/foo.txt and get confused that the file doesn't exist.

This change instead returns an error equivalent to ENOTDIR to the caller, which matches the behaviour of Node.js and Posix.

cc @KernelDeimos

[KernelDeimos]

This was causing problems in isomorphic-git, as it would readdir foo.txt, receive the listing of [ {path: foo.txt', ... } ], and then try to read foo.txt/foo.txt and get confused that the file doesn't exist.

I was really hoping I could come to the conclusion that we don't need two endpoints that stat filesystem nodes, but it looks like this elegance breaks other software so I'm going to merge this fix.

[AtkinsSJ]

I was really hoping I could come to the conclusion that we don't need two endpoints that stat filesystem nodes, but it looks like this elegance breaks other software so I'm going to merge this fix.

Yeah that would definitely be convenient! Maybe there's some other way of signalling that it's not a directory.