* Fix '/' and '..' not being valid mv targets
If `/` or `../` is specified as the destination for `mv`, it will fail with an error message saying it's not a valid destination. This fixes it to account for the fact that `Path::file_name` return `None` when the file name evaluates to `/` or `..`. It will only take the slow(er) path if `Path::file_name` returns `None` in its initial check.
Fixes#1291
* Add test
This improves incremental build time when working on what was previously
the root package. For example, previously all plugins would be rebuilt
with a change to `src/commands/classified/external.rs`, but now only
`nu-cli` will have to be rebuilt (and anything that depends on it).
