cmd/go: fix go get fail when GIT_TRACE set

GIT_TRACE write message to stderr, while run1 merge both stdout and
stderr. So function which call run1 and rely on its output will failed
to parse the result when run1 success.

By using cmd.Output(), we ensure only cmd standard out is returned.

Fixes #19682

Change-Id: I7002df17fe68aea1860ddc7382c68cc23548bd90
Reviewed-on: https://go-review.googlesource.com/126735
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/get/vcs.go

@@ -5,7 +5,6 @@
 package get
 
 import (
-	"bytes"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -428,19 +427,18 @@ func (v *vcsCmd) run1(dir string, cmdline string, keyval []string, verbose bool)
 		fmt.Printf("cd %s\n", dir)
 		fmt.Printf("%s %s\n", v.cmd, strings.Join(args, " "))
 	}
-	var buf bytes.Buffer
-	cmd.Stdout = &buf
-	cmd.Stderr = &buf
-	err = cmd.Run()
-	out := buf.Bytes()
+	out, err := cmd.Output()
 	if err != nil {
 		if verbose || cfg.BuildV {
 			fmt.Fprintf(os.Stderr, "# cd %s; %s %s\n", dir, v.cmd, strings.Join(args, " "))
-			os.Stderr.Write(out)
+			if ee, ok := err.(*exec.ExitError); ok && len(ee.Stderr) > 0 {
+				os.Stderr.Write(ee.Stderr)
+			} else {
+				fmt.Fprintf(os.Stderr, err.Error())
+			}
 		}
-		return out, err
 	}
-	return out, nil
+	return out, err
 }
 
 // ping pings to determine scheme to use.

src/cmd/go/testdata/script/get_with_git_trace.txt

@@ -0,0 +1,7 @@
+env GIT_TRACE=1
+
+[!net] skip
+[!exec:git] skip
+
+# go get should be success when GIT_TRACE set
+go get golang.org/x/text