various tweaks to code review.

various tweaks to code review.
main one is to check at submit time that
user name being used in checkin message
is listed in the CONTRIBUTORS file.
this should catch misconfigurations.

another is to cut the @domain part
from the R= and CC= lines on checkin
messages, so that cc'ing someone on
a change does not mean their email
address is recorded for all time.

R=r
CC=go-dev
http://go/go-review/1016036