go/printer: make //line formatting idempotent
go/printer: make //line formatting idempotent

Fixes "test.sh" (long test) in src/cmd/gofmt.

R=gri
CC=golang-dev
http://codereview.appspot.com/5307081

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