cmd/go: buffer output for go usage

It did tons of write syscalls before:
    https://www.youtube.com/watch?v=t60fhjAqBdw

This is the worst offender. It's not worth fixing all the cases of two
consecutive prints.

Change-Id: I95860ef6a844d89b149528195182b191aad8731b
Reviewed-on: https://go-review.googlesource.com/2371Reviewed-by: Rob Pike <r@golang.org>

src/cmd/go/main.go

@@ -5,6 +5,7 @@
 package main
 
 import (
+	"bufio"
 	"bytes"
 	"flag"
 	"fmt"
@@ -236,7 +237,9 @@ func capitalize(s string) string {
 }
 
 func printUsage(w io.Writer) {
-	tmpl(w, usageTemplate, commands)
+	bw := bufio.NewWriter(w)
+	tmpl(bw, usageTemplate, commands)
+	bw.Flush()
 }
 
 func usage() {