extract: shall borg warn if cwd is not empty?

Just read an old story on reddit, where somebody damaged their home directory by running:
```
borg extract REPO::archive
```

Well, that is designed to extract the archive contents to `.`, which usually should be a new/empty directory, just made for extraction purposes.

In that case, it was their normal home directory, so the archived files overwrote the current files - likely the archive was created using: `borg create REPO::archive .`

Shall borg extract warn and ask for confirmation if the cwd is not empty?

Start at the implementation of the `borg extract` command and inspect existing confirmation behavior and related tests. Determine how a non-empty current directory is detected and how a confirmation prompt should behave; done means extraction warns before proceeding and the behavior is covered by tests.

Written by the indexing model from the issue text.