gofmt: use os.Stdin instead of opening /dev/stdin
Opening /dev/stdin can sometimes fail. For example, in the acme editor,
executing "Edit ,|gofmt" fails with:
open /dev/stdin: no such device or address
Executing "Edit ,|ls -l /dev/stdin /proc/self/fd/0" gives:
lrwxrwxrwx 1 root root 15 2009-09-07 02:17 /dev/stdin -> /proc/self/fd/0
lrwx------ 1 fhs users 64 2009-11-26 22:05 /proc/self/fd/0 -> socket:[
5528230]
(This is my first change, and I've signed the individual contributor license agreement.)
R=rsc, gri
CC=golang-dev
https://golang.org/cl/162041