An artifact of the c2go translation was
a handful of instances of code like:

var s string
s += "foo"
return s

This CL converts those to simply 'return "foo"'.

The conversion was done mechanically with the
quick-and-dirty cleanup script at
https://gist.github.com/josharian/1fa4408044c163983e62.

I then manually moved a couple of comments in fmt.go.

toolstash -cmp thinks that there are no functional changes.

Change-Id: Ic0ebdd10f0fb8de0360a1041ce5cd10ae1168be9
Reviewed-on: https://go-review.googlesource.com/6265Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>