path.relative returns empty string when its from and to arguments are identical.

> path.relative(dir, dir)
''

Since '' is not a valid path, this can blow up:

function ls (dir) {
  dir = path.relative(process.cwd(), dir);
  console.log(dir);
  // ...

  console.log(fs.readdirSync(dir));
}

This function ls works most of the time (nonexistent files and permissions aside), but not if dir points to the working directory:

> ls('.')

Error: ENOENT: no such file or directory, scandir ''
    at Error (native)
    at Object.fs.readdirSync (fs.js:856:18)
    at ls (repl:6:16)

I find the fact that path.relative returns a valid path, except in one corner case when the two arguments are identical, rather inconvenient.

However, this is probably intentional since there is a test for this introduced in #2106. But I haven't found any reasoning for this, and the reasoning of #2106 itself is not (directly) applicable here: it's one thing to defensively treat empty strings as '.', and the other to return an empty string instead of a valid path.

Expected bahavior:

> path.relative(dir, dir)
'.'