sweet: only print git command stderr on failure

Currently a bunch of git helpers route git's stderr to Sweet's stderr,
but this just pollutes the output if everything worked fine. This change
captures the output and dumps it only on error.

Change-Id: I37e780e1deb98b44f13595fb8d53fd0568651de9
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/613955
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

Author: mknyszek

sweet/harnesses/common.go

@@ -5,6 +5,8 @@
 package harnesses
 
 import (
+	"bytes"
+	"fmt"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -17,37 +19,48 @@ func gitShallowClone(dir, url, ref string) error {
 	// Git 2.46+ has a global --no-advice flag, but that's extremely recent as of this writing.
 	cmd := exec.Command("git", "-c", "advice.detachedHead=false", "clone", "--depth", "1", "-b", ref, url, dir)
 	log.TraceCommand(cmd, false)
-	cmd.Stderr = os.Stderr
-	_, err := cmd.Output()
-	return err
+	var buf bytes.Buffer
+	cmd.Stderr = &buf
+	if _, err := cmd.Output(); err != nil {
+		return fmt.Errorf("git shallow clone: %v: stderr:\n%s", err, &buf)
+	}
+	return nil
 }
 
 func gitRecursiveCloneToCommit(dir, url, branch, hash string) error {
 	cloneCmd := exec.Command("git", "clone", "--recursive", "--shallow-submodules", "-b", branch, url, dir)
 	log.TraceCommand(cloneCmd, false)
-	cloneCmd.Stderr = os.Stderr
+	var buf bytes.Buffer
+	cloneCmd.Stderr = &buf
 	if _, err := cloneCmd.Output(); err != nil {
-		return err
+		return fmt.Errorf("git recursive clone: %v: stderr:\n%s", err, &buf)
 	}
+	buf.Reset()
 	checkoutCmd := exec.Command("git", "-C", dir, "-c", "advice.detachedHead=false", "checkout", hash)
 	log.TraceCommand(checkoutCmd, false)
-	checkoutCmd.Stderr = os.Stderr
-	_, err := checkoutCmd.Output()
-	return err
+	checkoutCmd.Stderr = &buf
+	if _, err := checkoutCmd.Output(); err != nil {
+		return fmt.Errorf("git checkout: %v: stderr:\n%s", err, &buf)
+	}
+	return nil
 }
 
 func gitCloneToCommit(dir, url, branch, hash string) error {
 	cloneCmd := exec.Command("git", "clone", "-b", branch, url, dir)
 	log.TraceCommand(cloneCmd, false)
-	cloneCmd.Stderr = os.Stderr
+	var buf bytes.Buffer
+	cloneCmd.Stderr = &buf
 	if _, err := cloneCmd.Output(); err != nil {
-		return err
+		return fmt.Errorf("git recursive clone: %v: stderr:\n%s", err, &buf)
 	}
+	buf.Reset()
 	checkoutCmd := exec.Command("git", "-C", dir, "-c", "advice.detachedHead=false", "checkout", hash)
 	log.TraceCommand(checkoutCmd, false)
-	checkoutCmd.Stderr = os.Stderr
-	_, err := checkoutCmd.Output()
-	return err
+	checkoutCmd.Stderr = &buf
+	if _, err := checkoutCmd.Output(); err != nil {
+		return fmt.Errorf("git checkout: %v: stderr:\n%s", err, &buf)
+	}
+	return nil
 }
 
 func copyFile(dst, src string) error {