tools: add commit message checks to ci-fairy
Useful for checking things like signed-off-by and some more cosmetic things that we expect in general from the commit message.
Current checks:
- s-o-b present or not (depending on commandline toggle)
- textwidth < 80 chars
- second line of commit message is an empty line
- no 'fixup!' or 'squash!' messages in the history
Fixes #9 (closed)