Add script to check Signed-off-by header in patches

Signed-off-by: Michal Čihař <mcihar@suse.cz>
Check Signed-off-by in pull requests

Signed-off-by: Michal Čihař <mcihar@suse.cz>
Better way to discover new commits

Signed-off-by: Michal Čihař <mcihar@suse.cz>