Add additional choices in --check-commit-status

This PR extends the `--check-commit-status` option implemented in #101 so that the following are now possible:

```
scc merge base --check-commit-status none
scc merge base --check-commit-status no-error
scc merge base --check-commit-status success-only
```

The `get_last commit()` and `get_last_status()` logic is also extended to retrieve these elements from either the head or the base repository by setting `ref` (default being to retrieve from the base repository). The same logic is applied to `set_commit_status()` /cc @knabar.

In `find_candidates()`, while checking the PR status, the last status is first queried on the base repository and if not existing, it is quiered on the head repository (allowing integration tests on snoopys-sandbox to pass since the status is set on the head repository).

The filter information is extended to expose the status filtering type and the Status-excluded PRs digest now includes the state of the excluded PRs.

Integration and unit tests have been reviewed to test these new functionalities.

/cc @hflynn