Remove constraint for compressing the root directory

[marcospb19]

Ouch emits an error when the user tries to compress the root directory.

This used to make sense when compressing was all done in-memory, however, it's been a long time since we do stream based compression, so compressing the root directory does not exhaust memory anymore (possible with .zip.gz, but we already have a warning and ask a question for that at runtime).

Now is just a problem of our implementation details:

- we cd into a parent dir before compressing a folder, but root directories do not have parent dir - function impl for reference