sanitize-commit: check for suid permissions on new files

Alongside misguided execute permissions, check for set*id permissions;
git won't propagate them, but there's no way they're a good sign.

Change-Id: Ie3499ec79fd275d6f9cd0cb331c9cf2165ba7949
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>