practically test the "from borg" importer

I have just added some code to import backups from a borg 1.x repo into a new borg 1.x repo.

```
borg-import borg -h # read this!
```

Importing an archive does:
- for each archive in the source repo
- extract archive to the extraction directory (subdirectory of cwd)
- create a new archive from the extracted data
- delete all files from the extraction directory

This is useful when there is some reason to rebuild the repo:
- use a new AES key (e.g. if your old key might be compromised)
- you can also transfer archives from multiple source repos into a single destination repo
- (add more reasons here)

There is a automated test that succeeds doing this, but more practical tests are definitely needed. Be careful!