codereview: really disable codereview when not available
codereview: really disable codereview when not available

$ hg p
codereview disabled: cannot open /Users/rsc/g/go/src/pkg/goplan9.googlecode.com/hg/lib/codereview/codereview.cfg
$

R=dsymonds, r
CC=golang-dev
http://codereview.appspot.com/1998046

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go