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

ssrl/go,dsriram/golang,gnanderson/go,ssrl/go,gnanderson/go,ssrl/go,gnanderson/go,tv42/old-go,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,tv42/old-go,dsriram/golang,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,tv42/old-go,ssrl/go,dsriram/golang,ssrl/go,ssrl/go,apparentlymart/golang,ssrl/go,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,tv42/old-go