Add option to UnrebasedPRs to check mismatching PR comments

This commit construct PR dictionaries based on parsed PR body/comments when
listing unrebased PRs. Once the PRs are listed, the from/to rebased comments
are checked against the other branch dictionary and mismatching candidates are
output unless the --no-check is passed to the command.
Refactor static check() method to allow method unit testing

- Move output printing to the notes() method
- Add unit tests for UnrebasedPRs command