Change: Replace `--repo2` option used by `init`/`copy` with `--from-repo`

The `init` and `copy` commands can read data from another repository.
However, confusingly `--repo2` referred to the repository *from* which the
`init` command copies parameters, but for the `copy` command `--repo2`
referred to the copy *destination*.

We've introduced a new option, `--from-repo`, which always refers to the
source repository for both commands. The old parameter names have been
deprecated but still work. To create a new repository and copy all snapshots
to it, the commands are now as follows:

```
restic -r /srv/restic-repo-copy init --from-repo /srv/restic-repo --copy-chunker-params
restic -r /srv/restic-repo-copy copy --from-repo /srv/restic-repo
```

https://github.com/restic/restic/pull/3742
https://forum.restic.net/t/5017
