Add extra granularite to get_last_commit() and get_last_status()

Allows to specify wether the commit and/or status should be retrieved
on the base repository or head repository.
Propagate new check-commit-status options to find_candidates()

- Add log information when parsing filters
- Extend exclusion condition depending on the filter and status
- Falls back to reading the head repo status if no status on the base repo
- Improve the status-excluded PRs digest by exposing the read status