autogen.sh: set format.subjectPrefix and sendemail.to if needed

Just set the rules automatically rather than asking each contributor to
update thing locally.

v2: Silence errors if run outside of git repo. (Eric)

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>