codereview: restrict sync to default branch
codereview: restrict sync to default branch

'default' is the name of the main branch,
the one that isn't a release branch.

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/4636072

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang