cmd/go: quote command line arguments in debug output

Debug output from go test -x may contain empty arguments.
This CL quotes arguments if needed. E.g. the output of
go test -x is now
  .../6g -o ./_go_.6 -p testmain -complete -D "" -I . -I $WORK ./_testmain.go
which is easier to grasp.

R=golang-dev, bradfitz, minux.ma, r
CC=golang-dev
https://golang.org/cl/8633043

Author: vdobler

src/cmd/go/build.go

@@ -1306,6 +1306,7 @@ func (b *builder) runOut(dir string, desc string, env []string, cmdargs ...inter
 
 // joinUnambiguously prints the slice, quoting where necessary to make the
 // output unambiguous.
+// TODO: See issue 5279. The printing of commands needs a complete redo.
 func joinUnambiguously(a []string) string {
 	var buf bytes.Buffer
 	for i, s := range a {